[COFF] Emit embedded -exclude-symbols: directives for hidden visibility for MinGW

This works with the automatic export of all symbols; in MinGW mode,
when a DLL has no explicit dllexports, it exports all symbols (except
for some that are hardcoded to be excluded, including some toolchain
libraries).

By hooking up the hidden visibility to the -exclude-symbols: directive,
the automatic export of all symbols can be controlled in an easier
way (with a mechanism that doesn't require strict annotation of every
single symbol, but which allows gradually marking more unnecessary
symbols as hidden).

The primary use case is dylib builds of LLVM/Clang. These can be done
in MinGW mode but not in MSVC mode, as MinGW builds can export all
symbols (and the calling code can use APIs without corresponding
dllimport directives). However, as all symbols are exported, it can
easily overflow the max number of exported symbols in a DLL (65536).

In the llvm-mingw distribution, only the X86, ARM and AArch64 backends
are enabled; for the LLVM 13.0.0 release, libLLVM-13.dll ended up with
58112 exported symbols. For LLVM 14.0.0, it was 62015 symbols. Current
builds of the 15.x branch end up at around 64650 symbols - i.e. extremely
close to the limit.

The msys2 packages of LLVM have had to progressively disable more
of their backends in their builds, to be able to keep building with a
dylib.

This allows improving the current mingw dylib situation significantly,
by using the same hidden visibility options and attributes as on Unix.
With those in place, a current build of LLVM git main ends up at 35142
symbols instead of 64650.

For code using hidden visibility, this now requires linking with either
a current git lld or ld.bfd. (Older lld error out on the unknown
directives, older ld.bfd will successfully link, but will print huge
amounts of warnings.)

Differential Revision: https://reviews.llvm.org/D130121

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