Demangle: avoid butchering parameter type

Demangle: avoid butchering parameter type

When demangling a CV-qualified function type with a final parameter with
a reference type, we would insert the CV qualification on the parameter
rather than the function, and in the process adjust the insertion point
by one extra, splitting the type name.  This avoids doing so, even
though the attribution is still incorrect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292965 91177308-0d34-0410-b5e6-96231b3b80d8