Itanium ABI: Mangle <mangled-name> according to the ABI

Itanium ABI: Mangle <mangled-name> according to the ABI

We attempted to be compatible with GCC's buggy mangling for templates
with a declaration for a template argument.

However, we weren't completely successful in copying their bug in cases
like:
  char foo;
  template <char &C> decltype(C) f() { return foo; };
  template char &f<foo>();

Instead, just follow the ABI specification.  This fixes PR22621.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@229644 91177308-0d34-0410-b5e6-96231b3b80d8