Teach SourceManager's content cache to keep track of whether its

Teach SourceManager's content cache to keep track of whether its
buffer was invalid when it was created, and use that bit to always set
the "Invalid" flag according to whether the buffer is invalid. This
ensures that all accesses to an invalid buffer are marked invalid,
improving recovery.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98690 91177308-0d34-0410-b5e6-96231b3b80d8