[clang] [C++20][Modules] Improve namespace look-up performance for modules. (PR #171769)

[Michael Park via cfe-commits]

mpark wrote:

> > The patch itself looks good. But I like to test it internally before merging. Hope I can make it in next week.
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> In our internal workloads, I didn't see noticeable changes in End-to-End build process. Maybe the reason is we've already done a so called "bottom up" build. But the good news is that I didn't find regression too. So LGTM.

Thanks for checking! Yeah, unless you have fine-grained modules with 1K+ modules declaring the same namespaces, I wouldn't expect to see any improvements with this.

For us, it's because at a certain scale with Thrift generated files, we end up with 1K+ modules imported that all define the `apache::thrift` namespace.

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> > And also I think it is worthy to add a test for it. We are able to test it with unittests. e.g., we can test the number of the return decls of `DeclContext::lookup`, or other similar words. We can add some API for testing such status too.
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> Would you like to add a test for this?  If you don't know how to do, you can look at clang/unittests/Serialization/LoadSpecLazilyTest.cpp for example. This is useful as lit test can't show all things.
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I'll be working on the test tomorrow! Thanks for the pointer.


https://github.com/llvm/llvm-project/pull/171769