[clang-tools-extra] [clang-tidy] Fix loading clang-tidy as a Clang frontend plugin (PR #221961)

[Zeyi Xu via cfe-commits]

zeyi2 wrote:

Some manual testing for this PATCH:

Linux (Tested on ampere1 AArch64 Server):

```
llvm-lit: /home/xuzy/llvm-testing/pr-221961-50b42f202c13/source/llvm/utils/lit/lit/llvm/config.py:569: note: using clang: /home/xuzy/llvm-testing/pr-221961-50b42f202c13/build/bin/clang
-- Testing: 3 tests, 3 workers --
PASS: Clang Tools :: clang-tidy/CTTestTidyModule.cpp (1 of 3)
PASS: Clang Tools :: clang-tidy/infrastructure/clang-plugin-analyzer.cpp (2 of 3)
PASS: Clang Tools :: clang-tidy/infrastructure/clang-plugin.cpp (3 of 3)

Testing Time: 0.28s

Total Discovered Tests: 3
  Passed: 3 (100.00%)
```

MacOS (Tested on my Macbook Air):

```
llvm-lit: /Users/zeyi2/Documents/projects/llvm-project/llvm/utils/lit/lit/llvm/config.py:569: note: using clang: /Users/zeyi2/Documents/projects/llvm-project/build-tidy-plugin/bin/clang
-- Testing: 3 tests, 3 workers --
PASS: Clang Tools :: clang-tidy/CTTestTidyModule.cpp (1 of 3)
PASS: Clang Tools :: clang-tidy/infrastructure/clang-plugin.cpp (2 of 3)
PASS: Clang Tools :: clang-tidy/infrastructure/clang-plugin-analyzer.cpp (3 of 3)

Testing Time: 0.89s

Total Discovered Tests: 3
  Passed: 3 (100.00%)
```

Windows: MinGW, MSVC, clang-cl all failed.
- https://github.com/zeyi2/llvm-testing/actions/runs/34334835215/job/102411698571
- https://github.com/zeyi2/llvm-testing/actions/runs/34333356395
- https://github.com/zeyi2/llvm-testing/actions/runs/34333791439

And they are already broken on trunk. So I'll disable support for Windows for now.


https://github.com/llvm/llvm-project/pull/221961