[demangler] Reorder parseNestedName loop

parseNestedName's main loop allowed parsing a grammar that was more
flexible than the actual grammar.  This refactors that to rule out
some more incorrect manglings.

1) The 'L' extension only applies to unqualified-name components, so
check it just there.

2) The 'M' suffix is, AFAICT, removed from the grammar.  Rather than
eliminate it, let's parse it after we've parsed a component.

Added some additional bad mangling tests, which are now rejected.

I don't break the 'T' and 'D[tT]' cases out of the loop, even though
they can only appear at first position, as it seems simpler to just
check there is nothing SoFar.

Reviewed By: ChuanqiXu

Differential Revision: https://reviews.llvm.org/D119542