Remove the last of the old-style Preprocessor::Diag methods.

include/clang/Lex/Preprocessor.h

lib/Lex/LiteralSupport.cpp

lib/Lex/PPDirectives.cpp

lib/Lex/PPExpressions.cpp

lib/Lex/Preprocessor.cpp

lib/Lex/PreprocessorLexer.cpp

lib/Lex/TokenLexer.cpp

lib/Parse/ParsePragma.cpp