Set an android-specific patch level to Clang

Bug: http://b/28196329
Bug: http://b/37574207

Set a custom patch level for LLVM and Clang.  This will be used to
- identify Android Clang versions (as opposed to using non-contiguous
build numbers)
- version-match Clang profdata files for PGO

Test: python toolchain/llvm/android/build.py
Change-Id: I314de350b0a6287632c7b3c4d5a29c84c6219c7b