[bazel] Add missing clang/unittests targets (#150491)

Mostly just adding tests that are in clang/unittests/CMakeLists.txt, but
there a couple other cleanups here:
* Add `directory_watcher` targets in clang so we can add tests for it
* Add some library targets (e.g. `dataflow_testing_support`) for
downstream use (e.g. unit tests outside of LLVM that use these headers
in tests)
* Remove `rename_tests_tooling_hdrs` which has been unused since
`rename_tests` itself was deleted

The InterpreterTest target added here passes on our internal CI, but
fails when I run it w/ bazel for some reason:
```
[ RUN      ] InterpreterTest.InstantiateTemplate
JIT session error: Symbols not found: [ __clang_Interpreter_SetValueWithAlloc, _ZnwmPv26__clang_Interpreter_NewTag ]
Failure value returned from cantFail wrapped call
Failed to materialize symbols: { (main, { __orc_init_func.incr_module_133, $.incr_module_133.__inits.0 }) }
```
I'd like to figure out what's going on, but for now I just excluded it
w/ a test filter

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