[demangler] Fix undocumented Local encoding

GCC emits [some] static symbols with an 'L' mangling, which we attempt
to demangle.  But the module mangling changes have exposed that we
were doing so at the wrong level.  Such manglings are outside of the
ABI as they are internal-linkage, so a bit of reverse engineering was
needed.  This adjusts the demangler along the same lines as the
existing gcc demangler (which is not yet module-aware).  'L' is part
of an unqualified name.  As before we merely parse the 'L', and then
ignore it.

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D123138