Sema: Fix explicit address space cast in C++

Sema: Fix explicit address space cast in C++

Currently clang does not allow implicit cast of a pointer to a pointer type
in different address space but allows C-style cast of a pointer to a pointer
type in different address space. However, there is a bug in Sema causing
incorrect Cast Expr in AST for the latter case, which in turn results in
invalid LLVM IR in codegen.

This is because Sema::IsQualificationConversion returns true for a cast of
pointer to a pointer type in different address space, which in turn allows
a standard conversion and results in a cast expression with no op in AST.

This patch fixes that by let Sema::IsQualificationConversion returns false
for a cast of pointer to a pointer type in different address space, which
in turn disallows standard conversion, implicit cast, and static cast.
Finally it results in an reinterpret cast and correct conversion kind is set.

Differential Revision: https://reviews.llvm.org/D49294

llvm-svn: 337540