[modules] Fix incorrect diagnostic mapping computation when a module changes

diagnostic settings using _Pragma within a macro.

The AST writer had previously been assuming that all diagnostic state
transitions would occur within a FileID corresponding to a file. When a
diagnostic state change occured within a macro, it was unable to form a
location for that state change and would instead corrupt the diagnostic state
of the "root" node (and thus that of the main compilation).

Also introduce a "#pragma clang __debug diag_mapping" debugging utility
that I added to track this issue down.

llvm-svn: 324695