implement basic support for __label__.  I wouldn't be shocked if there are
bugs from other clients that don't expect to see a LabelDecl in a DeclStmt,
but if so they should be easy to fix.

This implements most of PR3429 and rdar://8287027



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125817 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed