Add some helper functions to the demangle utility classes.

Add some helper functions to the demangle utility classes.

These are all methods that, while not currently used in the
Itanium demangler, are generally useful enough that it's
likely the itanium demangler could find a use for them.  More
importantly, they are all necessary for the Microsoft demangler
which is up and coming in a subsequent patch.  Rather than
combine these into a single monolithic patch, I think it makes
sense to commit this utility code first since it is very simple,
this way it won't detract from the substance of the MS demangler
patch.

llvm-svn: 337316