Give Type::isIntegralType() an ASTContext parameter, so that it

provides C "integer type" semantics in C and C++ "integral type"
semantics in C++. 

Note that I still need to update isIntegerType (and possibly other
predicates) using the same approach I've taken for
isIntegralType(). The two should have the same meaning, but currently
don't (!).

llvm-svn: 106074