Add flag -fauto-profile as alias to -fprofile-sample-use.

Summary:
GCC uses -fauto-profile to enable sample-based PGO. This patch
adds it to Clang as an alias for -fprofile-sample-use.

Differential Revision: http://llvm-reviews.chandlerc.com/D2353

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196589 91177308-0d34-0410-b5e6-96231b3b80d8