[MS ABI] Fix C++ mangling references to declarations.

Several issues have been discovered and (hopefully) fixed here:

- Reference NTTPs should be mangled in the same manner as pointer
ones.

-  Pointer fields of class type NTTPs should be treated in the same
manner as reference ones.

- Pointer-to-member fields of class type NTTPs should be treated
differently compared to pointer-to-member NTTPs. Tests on
pointer-to-member-function NTTP class fields added.

- Correct mangling of pointers to anonymous union members.

- A bug in mangling references to subobjects fixed.

- Mangling array subscripts and base class members in references
to subobjects.

Reference NTTP mangling was done back in 2013
in e8fdc06e, and Microsoft might change
mangling algorithm since then. But class type NTTPs are introduced only
in C++20, and the test was written in
b637148e.
It is strange if the MS ABI had been realy changed, because Microsoft
claims that they mainta...