[Clang] Add explicit visibility symbol macros (#108276)

This is part of the effort to support for enabling plugins on windows by
adding better support for building llvm and clang as a DLL. These macros
are similar to the ones i added in #96630, but are for clang.
Added explicit symbol visibility macros definitions that are defined in
a new header and will be used to for shared library builds of clang to
export
symbols. 
Updated clang cmake to define a macro to enable the symbol visibility
macros and explicitly disable them for clang tools that always use
static linking.

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Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

0 files changed, 0 insertions, 0 deletions