[MS Demangler] Refactor some of the name parsing code.

There are some very subtle differences between how one should
parse symbol names and type names.  They differ with respect
to back-referencing, the set of legal values that can appear
as the unqualified portion, and various other aspects.

By separating the parsing code into separate paths, we can
remove a lot of ambiguity during the demangling process, which
is necessary for demangling more complicated things like
function local statics, nested classes, and lambdas.

llvm-svn: 338207