Add more Parser/Sema support for GCC asm-label extension.

 - ActOnDeclarator now takes an additional parameter which is the
   AsmLabel if used. Its unfortunate that this bubbles up this high,
   but we cannot just lump it in as an attribute without mistakenly
   *accepting* it as an attribute.
 - The actual asm-label itself is, however, encoded as an AsmLabelAttr
   on the FunctionDecl.
 - Slightly improved parser error recovery on malformed asm-labels.
 - CodeGen support still missing...

llvm-svn: 54339