Fix a longstanding (but previously unknown) bug in the lazy

deserialization of precompiled headers, where the deserialization of
the source location entry for a buffer (e.g., macro instantiation
scratch space) would overwrite a one-element FileID cache in the
source manager. When tickled at the wrong time, we would return the
wrong decomposed source location and eventually cause c-index-test to
crash.

Found by dumb luck. It's amazing this hasn't shown up before.

llvm-svn: 98940