tools/clang: compile clang tools into the binary

Compiled clang tools into Go binaries using cgo.
This significantly simplifies building and deployment.
This also enables unit testing of clang tools.
Now raw go test for clang tools will build them, run,
and verify output.

Each clang tool is still started as a subprocess.
I've experimented with running them in-process,
but this makes stdout/stderr interception extremly complicated,
and it seems that clang tools still use unsynchronized global state,
which breaks when invoked multiple times.
Subprocesses also make it safer in the face of potential memory leaks,
or memory corruptions in clang tools.

Fixes #6645

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