Specifically disallow primary-expressions of the form LT_... on the basis of...

Specifically disallow primary-expressions of the form LT_... on the basis of this decition: http://sourcerytools.com/pipermail/cxx-abi-dev/2011-August/002422.html .  Failure to disallow this was causing infinite recursion in the demangler when these symbols show up due to mangling bugs.  This patch causes the demangler to return an invalid mangled name result rather than crash in infinite recursion.

llvm-svn: 163859