[MS Demangler] Properly handle function parameter back-refs.

[MS Demangler] Properly handle function parameter back-refs.

Properly demangle function parameter back-references.

Previously we treated lists of function parameters and template
parameters the same. There are some important differences with regards
to back-references, and some less important differences regarding which
characters can appear before or after the name.

The important differences are that with a given type T, all instances of
a function parameter list share the same global back-ref table.
Specifically, if X and Y are function pointers, then there are 3
entities in the declaration X func(Y) which all affect and are affected
by the master parameter back-ref table:
  1) The parameter list of X's function type
  2) the parameter list of func itself
  3) The parameter list of Y's function type.

The previous code would create a back-reference table that was local to
a single parameter list, so it would not be shared across parameter
lists.

This was discovered when porting ms-back-references.test from clang's
mangling tests. All of these tests should now pass with the new changes.

In doing so, I split the function for parsing template and function
parameters into two separate functions. This makes the template
parameter list parsing code in particular very small and easy to
understand now.

Differential Revision: https://reviews.llvm.org/D49875

llvm-svn: 338075