ASTUnit::getMainBufferWithPrecompiledPreamble: use MD5 hash of the remapped

ASTUnit::getMainBufferWithPrecompiledPreamble: use MD5 hash of the remapped
files to tell if they were changed since the last time we have computed the
preamble

We used to check only the buffer size, so if the new remapped buffer has the
same size as the previous one, we would think that the buffer did not change,
and we did not rebuild the preambule, which sometimes caused us to crash.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197755 91177308-0d34-0410-b5e6-96231b3b80d8