[Preprocessor] Fix crash emitting note with framework location for "file not found...

[Preprocessor] Fix crash emitting note with framework location for "file not found" error.

A filename can be remapped with a header map to point to a framework
header and we can find the corresponding framework without the header.
But if the original filename doesn't have a remapped framework name,
we'll fail to find its location and will dereference a null pointer
during diagnostics emission.

Fix by tracking remappings better and emit the note only if a framework
is found before any of the remappings.

rdar://problem/48883447

Reviewers: arphaman, erik.pilkington, jkorous

Reviewed By: arphaman

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D61707

llvm-svn: 361779