[libclang] Store unsaved file hashes when recording parsing invocations

Storing the contents of unsaved files is too expensive.
Instead a hash is stored with a record invocation. When a reproducer is
generated, Clang will compare the stored hashes to the new hashes to determine
if the contents of a file has changed. This way we'll know when a reproducer was
generated for a different source to the one that triggered the original crash.

rdar://35322543

llvm-svn: 319729