ms-inline-asm: Add a sema callback for looking up label names

The implementation of the callback in clang's Sema will return an
internal name for labels.

Test Plan: Will be tested in clang.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4587
ms-inline-asm: Scope inline asm labels to functions

Summary:
This fixes PR20023.  In order to implement this scoping rule, we piggy
back on the existing LabelDecl machinery, by creating LabelDecl's that
will carry the "internal" name of the inline assembly label, which we
will rewrite the asm label to.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D4589