[LLVM-3.9] Specify that we are using the legacy interface
[LLVM-3.9] Specify that we are using the legacy interface

LLVM pass manager infrastructure is currently getting rewritten to be
more flexible, but the rewrite isn't complete yet.

aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust