C++: Tests: Reset diagnostic messages
C++: Tests: Reset diagnostic messages

...otherwise they add up and subsequent tests checking for the error count will
fail:

FAIL!  : tst_AST::objc_try_statement_1() Compared values are not the same
   Actual   (diag.errorCount): 5
   Expected (0): 0
   Loc: [/home/nikolai/dev/creator/qtcreator-master/tests/auto/cplusplus/ast/tst_ast.cpp(1400)]
...

Change-Id: I8b4356331e95dceb1e99bb26e6ebf4591d70a08c
Reviewed-by: David Schulz <f07954e8cc0c222cd136162588d217f330598215@digia.com>

omniacreator/qtcreator,kuba1/qtcreator,farseerri/git_code,Distrotech/qtcreator,amyvmiwei/qt-creator,maui-packages/qt-creator,kuba1/qtcreator,AltarBeastiful/qt-creator,amyvmiwei/qt-creator,kuba1/qtcreator,AltarBeastiful/qt-creator,amyvmiwei/qt-creator,Distrotech/qtcreator,danimo/qt-creator,xianian/qt-creator,danimo/qt-creator,farseerri/git_code,martyone/sailfish-qtcreator,martyone/sailfish-qtcreator,richardmg/qtcreator,amyvmiwei/qt-creator,danimo/qt-creator,farseerri/git_code,xianian/qt-creator,martyone/sailfish-qtcreator,Distrotech/qtcreator,AltarBeastiful/qt-creator,darksylinc/qt-creator,AltarBeastiful/qt-creator,darksylinc/qt-creator,AltarBeastiful/qt-creator,kuba1/qtcreator,richardmg/qtcreator,AltarBeastiful/qt-creator,Distrotech/qtcreator,colede/qtcreator,maui-packages/qt-creator,farseerri/git_code,darksylinc/qt-creator,danimo/qt-creator,farseerri/git_code,richardmg/qtcreator,omniacreator/qtcreator,Distrotech/qtcreator,colede/qtcreator,amyvmiwei/qt-creator,colede/qtcreator,maui-packages/qt-creator,martyone/sailfish-qtcreator,darksylinc/qt-creator,darksylinc/qt-creator,richardmg/qtcreator,richardmg/qtcreator,colede/qtcreator,colede/qtcreator,martyone/sailfish-qtcreator,farseerri/git_code,darksylinc/qt-creator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,omniacreator/qtcreator,farseerri/git_code,Distrotech/qtcreator,xianian/qt-creator,richardmg/qtcreator,omniacreator/qtcreator,amyvmiwei/qt-creator,kuba1/qtcreator,xianian/qt-creator,danimo/qt-creator,maui-packages/qt-creator,kuba1/qtcreator,colede/qtcreator,martyone/sailfish-qtcreator,xianian/qt-creator,xianian/qt-creator,maui-packages/qt-creator,kuba1/qtcreator,Distrotech/qtcreator,amyvmiwei/qt-creator,danimo/qt-creator,colede/qtcreator,omniacreator/qtcreator,farseerri/git_code,omniacreator/qtcreator,kuba1/qtcreator,darksylinc/qt-creator,richardmg/qtcreator,martyone/sailfish-qtcreator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,danimo/qt-creator,maui-packages/qt-creator,xianian/qt-creator,kuba1/qtcreator,xianian/qt-creator,danimo/qt-creator,danimo/qt-creator,maui-packages/qt-creator,darksylinc/qt-creator,omniacreator/qtcreator