[Microsoft][C++] Clang doesn't support a use of "this" pointer inside inline asm

Clang doesn’t support a use of “this” pointer inside inline asm.
When I tried to compile a class or a struct (see example) with an inline asm that contains "this" pointer.
Clang returns with an error.
This patch fixes that.

error: expected unqualified-id
For example:
'''
struct A {
    void f() {
          __asm mov eax, this
              // error: expected unqualified-id
                  }
};
'''
Differential Revision: http://reviews.llvm.org/D15115

llvm-svn: 255645