[clang][cmake] Use `GNUInstallDirs` to support custom installation dirs

[Ericson2314]

Summary

I am breaking apart D99484 so the cause of build failures is easier to
understand.

[Ericson2314]

1. Updating D117419: [clang][cmake] Use GNUInstallDirs to support custom installation dirs #
2. Enter a brief description of the changes included in this update.
3. The first line is used as subject, next lines as comment. #
4. If you intended to create a new revision, use:
5. $ arc diff --create

Add missing import

[Ericson2314]

Fix bug

[beanz]

clang/CMakeLists.txt

3

If GNUInstallDirs is included before the project call enables languages it results in a CMake warning being logged.

[Ericson2314]

clang/CMakeLists.txt

3

See D117639 where we are discussing this problem and what to do about it.

[Ericson2314]

Avoid warning

[Ericson2314]

Remove the LLVM part, which is now D101070, and furthermore known to cause issues.

[Ericson2314]

I will land this next as part of my great D99484 bisect. Now that I have in fact found the issue, I am more confident this part, which is analogous to all the parts that have gone in fine, is harmless.