[Fix] Speedup -Wunsafe-buffer-usage when using clang modules. (#127161) Each piece of code should have analysis run on it precisely once. However, if you build a module, and then build another module depending on it, the header file of the module you depend on will have `-Wunsafe-buffer-usage` run on it twice. This normally isn't a huge issue, but in the case of using the standard library as a module, simply adding the line `#include <cstddef>` increases compile times by 900ms (from 100ms to 1 second) on my machine. I believe this is because the standard library has massive modules, of which only a small part is used (the AST is ~700k lines), and because if what I've been told is correct, the AST is lazily generated, and `-Wunsafe-buffer-usage` forces it to be evaluated every time. See https://issues.chromium.org/issues/351909443 for details and benchmarks.
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