Fix the rest of rdar://6243860 hopefully. This requires changing FileIDInfo

to whether the fileid is a 'extern c system header' in addition to whether it
is a system header, most of this is spreading plumbing around.  Once we have that,
PPLexerChange bases its "file enter/exit" notifications to PPCallbacks to
base the system header state on FileIDInfo instead of HeaderSearch.  Finally,
in Preprocessor::HandleIncludeDirective, mirror logic in GCC: the system headerness
of a file being entered can be set due to the #includer or the #includee.

llvm-svn: 56688