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This change adds the HAS_CPP_ATTRIBUTE and NO_UNIQUE_ADDRESS macros to
base/compiler_specific.h. This allows using the C++20 no_unique_address
attribute in prior dialects if the compiler supports it.

Bug: None
Change-Id: I6aff82e7b195f5eaf93551049c9dfa6a3fd1fbc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510396Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822768}