Rejigger how -pedantic and -pedantic-errors work and their interaction

with other diagnostic mapping.  In the new scheme, -Wfoo or -Wno-foo or 
-Werror=foo all override the -pedantic options, and __extension__ 
robustly silences all extension diagnostics in their scope.

An added bonus of this change is that MAP_DEFAULT goes away, meaning that
per-diagnostic mapping information can now be stored in 2 bits, doubling
the density of the Diagnostic::DiagMapping array.  This also 
substantially simplifies Diagnostic::getDiagnosticLevel.

OTOH, this temporarily introduces some "macro intensive" code in 
Diagnostic.cpp.  This will be addressed in a later patch.

llvm-svn: 69154