Initial commit for static if feature. Currently only parsing
and some sema analysis are added.
	modified:   include/clang/Basic/DiagnosticSemaKinds.td
	modified:   include/clang/Parse/Parser.h
	modified:   include/clang/Sema/Sema.h
	modified:   lib/Parse/ParseDecl.cpp
	modified:   lib/Parse/ParseDeclCXX.cpp
	modified:   lib/Sema/SemaDeclCXX.cpp