[UpdateCompilerUsed] API rename and cleanup, suggested by Rafaael.

* UpdateCompilerUsed() -> updateCompilerUsed()
* ThinLTO doesn't use the API so we can remove the include
* Clean up unused #include <functional> from the header
* Rename #ifdef guard comment to be correct.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273461 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed