Clang support for -flto=thin.

Summary:
Add clang support for -flto=thin option, which is used to set the
EmitFunctionSummary code gen option on compiles.

Add -flto=full as an alias to the existing -flto.

Add tests to check for proper overriding of -flto variants on the
command line, and convert grep tests to FileCheck.

Reviewers: dexonsmith, joker.eph

Subscribers: davidxl, cfe-commits

Differential Revision: http://reviews.llvm.org/D11908

llvm-svn: 250398