Some FileIDs that may be used by PlistDiagnostics were not added while building
a list of pieces. This caused assertion violation in GetFID() function.
This patch adds some missing FileIDs to avoid the assertion. It also contains
small refactoring of PlistDiagnostics::FlushDiagnosticsImpl().
Patch by Aleksei Sidorin, Ilya Palachev.
Differential Revision: https://reviews.llvm.org/D22090
llvm-svn: 280360