Fix compile warnings in CxxToolFlags.
Fix compile warnings in CxxToolFlags.

Summary:
Fixing the following 'ant compile' warnings due to return type mismatch in overriding class.

  compile:
    [javac] Compiling 2140 source files to /Users/ruibm/buck/build/classes
    [javac] 1. WARNING: Annotation Value.Parameter is superfluous when Style(allParameters = true)
    [javac] 2. WARNING in /Users/ruibm/buck/src/com/facebook/buck/cxx/CxxToolFlags.java (at line 91)
    [javac]   abstract class AbstractExplicitCxxToolFlags extends CxxToolFlags {
    [javac]                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    [javac] Generated 'Builder.from' method will not copy from attribute 'platformFlags' because it has different return type in supertype (And we cannot handle generic specialization or covarian overrides yet). Sometimes it is possible to avoid this by providing abstract override method in this value object
    [javac] ----------
    [javac] 3. WARNING in /Users/ruibm/buck/src/com/facebook/buck/cxx/CxxToolFlags.java (at line 91)
    [javac]   abstract class AbstractExplicitCxxToolFlags extends CxxToolFlags {
    [javac]                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    [javac] Generated 'Builder.from' method will not copy from attribute 'ruleFlags' because it has different return type in supertype (And we cannot handle generic specialization or covarian overrides yet). Sometimes it is possible to avoid this by providing abstract override method in this value object
    [javac] 3 problems (3 warnings)

Test Plan: ci

Reviewed By: Coneko

fbshipit-source-id: 58689c3

ilya-klyuchnikov/buck,marcinkwiatkowski/buck,dsyang/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,k21/buck,shybovycha/buck,shybovycha/buck,nguyentruongtho/buck,dsyang/buck,clonetwin26/buck,SeleniumHQ/buck,romanoid/buck,dsyang/buck,k21/buck,k21/buck,ilya-klyuchnikov/buck,romanoid/buck,dsyang/buck,clonetwin26/buck,romanoid/buck,k21/buck,nguyentruongtho/buck,clonetwin26/buck,LegNeato/buck,ilya-klyuchnikov/buck,shs96c/buck,Addepar/buck,robbertvanginkel/buck,brettwooldridge/buck,robbertvanginkel/buck,dsyang/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,shs96c/buck,brettwooldridge/buck,brettwooldridge/buck,romanoid/buck,shs96c/buck,shs96c/buck,rmaz/buck,rmaz/buck,clonetwin26/buck,zhan-xiong/buck,facebook/buck,marcinkwiatkowski/buck,nguyentruongtho/buck,zpao/buck,Addepar/buck,rmaz/buck,zhan-xiong/buck,zhan-xiong/buck,rmaz/buck,LegNeato/buck,brettwooldridge/buck,marcinkwiatkowski/buck,nguyentruongtho/buck,JoelMarcey/buck,shs96c/buck,shybovycha/buck,LegNeato/buck,rmaz/buck,rmaz/buck,dsyang/buck,shybovycha/buck,ilya-klyuchnikov/buck,LegNeato/buck,dsyang/buck,zpao/buck,brettwooldridge/buck,shs96c/buck,LegNeato/buck,zpao/buck,robbertvanginkel/buck,clonetwin26/buck,zhan-xiong/buck,zhan-xiong/buck,Addepar/buck,shybovycha/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,JoelMarcey/buck,zhan-xiong/buck,JoelMarcey/buck,k21/buck,robbertvanginkel/buck,dsyang/buck,romanoid/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,SeleniumHQ/buck,facebook/buck,dsyang/buck,SeleniumHQ/buck,robbertvanginkel/buck,clonetwin26/buck,Addepar/buck,clonetwin26/buck,zhan-xiong/buck,marcinkwiatkowski/buck,shybovycha/buck,SeleniumHQ/buck,JoelMarcey/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,clonetwin26/buck,shs96c/buck,shs96c/buck,brettwooldridge/buck,facebook/buck,dsyang/buck,k21/buck,brettwooldridge/buck,JoelMarcey/buck,SeleniumHQ/buck,romanoid/buck,rmaz/buck,LegNeato/buck,facebook/buck,zpao/buck,rmaz/buck,brettwooldridge/buck,robbertvanginkel/buck,shybovycha/buck,clonetwin26/buck,zhan-xiong/buck,LegNeato/buck,Addepar/buck,JoelMarcey/buck,rmaz/buck,JoelMarcey/buck,robbertvanginkel/buck,Addepar/buck,zpao/buck,marcinkwiatkowski/buck,kageiit/buck,dsyang/buck,LegNeato/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,LegNeato/buck,robbertvanginkel/buck,SeleniumHQ/buck,romanoid/buck,zhan-xiong/buck,kageiit/buck,robbertvanginkel/buck,kageiit/buck,JoelMarcey/buck,SeleniumHQ/buck,Addepar/buck,LegNeato/buck,k21/buck,k21/buck,k21/buck,SeleniumHQ/buck,JoelMarcey/buck,SeleniumHQ/buck,shs96c/buck,rmaz/buck,marcinkwiatkowski/buck,romanoid/buck,shybovycha/buck,shybovycha/buck,clonetwin26/buck,k21/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,kageiit/buck,romanoid/buck,brettwooldridge/buck,dsyang/buck,ilya-klyuchnikov/buck,marcinkwiatkowski/buck,facebook/buck,shs96c/buck,romanoid/buck,marcinkwiatkowski/buck,zpao/buck,kageiit/buck,kageiit/buck,nguyentruongtho/buck,robbertvanginkel/buck,SeleniumHQ/buck,rmaz/buck,Addepar/buck,romanoid/buck,JoelMarcey/buck,shs96c/buck,romanoid/buck,zpao/buck,shs96c/buck,LegNeato/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,zhan-xiong/buck,clonetwin26/buck,dsyang/buck,Addepar/buck,LegNeato/buck,clonetwin26/buck,shybovycha/buck,shybovycha/buck,SeleniumHQ/buck,k21/buck,marcinkwiatkowski/buck,brettwooldridge/buck,Addepar/buck,Addepar/buck,brettwooldridge/buck,shybovycha/buck,Addepar/buck,marcinkwiatkowski/buck,shs96c/buck,LegNeato/buck,JoelMarcey/buck,romanoid/buck,marcinkwiatkowski/buck,clonetwin26/buck,rmaz/buck,JoelMarcey/buck,facebook/buck,shybovycha/buck,facebook/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,Addepar/buck,k21/buck,brettwooldridge/buck,kageiit/buck,SeleniumHQ/buck,nguyentruongtho/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,k21/buck,rmaz/buck