[flang][MSVC] Use list<Message> rather than forward_list<> in Messages

The implementation of Messages with forward_list<> makes some
nonstandard assumptions about the validity of iterators that don't
hold up with MSVC's implementation.  Use list<> instead.  The
measured performance is comparable.

This change obviated a distinction between two member functions
of Messages, and the uses of one have been replaced with calls
to the other.

Similar usage in CharBuffer was also replaced for consistency.

Differential revision: https://reviews.llvm.org/D91210