Add -fbuild-session-file as an alternative to -fbuild-session-timestamp

Add -fbuild-session-file as an alternative to -fbuild-session-timestamp

Build systems tend to traffic in files and modification times, so having
them touch a file at the beginning of the build can be easier than
having them update the compile command they use every time they build.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214577 91177308-0d34-0410-b5e6-96231b3b80d8