Add Builtins.def attribute for "can be a constant expression".

 - Enabled for builtins which are always constant expressions
   (__builtin_huge_val*, __builtin_inf*, __builtin_constant_p,
   __builtin_classify_type, __builtin___CFStringMakeConstantString).

Added Builtin::Context::isConstantExpr.
 - Currently overly simply interface which only works for builtins
   whose constantexprness does not depend on their arguments.

CallExpr::isBuiltinConstantExpr now takes an ASTContext argument.

llvm-svn: 56983