llvm-undname: Make demangling of MD5 names more robust

Demangler::parse() for MD5 names would:

1. Put all remaining text into the MD5 name sight unseen
2. Not modify MangledName

This meant that if the demangler recursively called parse() (e.g. in
demangleLocallyScopedNamePiece()), every recursive call that started on
an MD5 name would add all remaining bytes to the output buffer but
only advance the input by a byte.  For valid inputs, MD5 types are
never (well, see comments for 2 exceptions) nested, but for invalid
input this could cause memory use quadratic in the input size.

llvm-svn: 361744