Allow DW_ATE_UTF for Rust characters

Allow DW_ATE_UTF for Rust characters

The Rust compiler plans to change the encoding of a Rust 'char' type
to use DW_ATE_UTF.  You can see the discussion here:

    https://github.com/rust-lang/rust/pull/89887

However, this fails in gdb.  I looked into this, and it turns out that
the handling of DW_ATE_UTF is currently fairly specific to C++.  In
particular, the code here assumes the C++ type names, and it creates
an integer type.

This comes from commit 53e710acd ("GDB thinks char16_t and char32_t
are signed in C++").  The message says:

    Both places need fixing.  But since I couldn't tell why dwarf2read.c
    needs to create a new type, I've made it use the per-arch built-in
    types instead, so that the types are only created once per arch
    instead of once per objfile.  That seems to work fine.

... which is fine, but it seems to me that it's also correct to make a
new character type; and this approach is better because it preserves
the type name as well.  This does use more memory, but first we
shouldn't be too concerned about the memory use of types coming from
debuginfo; and second, if we are, we should implement type interning
anyway.

Changing this code to use a character type revealed a couple of
oddities in the C/C++ handling of TYPE_CODE_CHAR.  This patch fixes
these as well.

I filed PR rust/28637 for this issue, so that this patch can be
backported to the gdb 11 branch.