[clang] [clang][Sema] Shard out SemaTests into their own binary (PR #155548)

[Reid Kleckner via cfe-commits]

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@@ -1,4 +1,7 @@
-add_clang_unittest(SemaTests
+# There are only a small number of tests here,
+# allow running them without building all of
+# AllClangUnitTests.
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rnk wrote:

If you look at the list of transitive dependencies below, I expect the SemaTests binary will be quite large relative to `clang`, like about `30%`. How large is it? Do you think that the additional productivity benefit of having this small granularity unit test warrants the increased link costs to the project?

I could be convinced this is the right tradeoff, but I'd like to have that information so we can make the right tradeoff.

https://github.com/llvm/llvm-project/pull/155548