[clang] [clang][DependencyScanning] Add Test Coverage of `StabeDirs` during By-Name Lookups (PR #168143)

[Qiongsi Wu via cfe-commits]

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+// UNSUPPORTED: system-windows
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qiongsiwu wrote:

Good question! 

The stable dir tests fail on Windows most likely due to the sed invocation. So the later suggestion might have enabled it if `clang-scan-deps` were a bit more forgiving. 

The existing stable dir tests are marked unsupported on Windows (see [here](https://github.com/llvm/llvm-project/blob/5407e62611abfbb359f595d89d9f29adf647be02/clang/test/ClangScanDeps/modules-in-stable-dirs.c#L2) and [here](https://github.com/llvm/llvm-project/blob/5407e62611abfbb359f595d89d9f29adf647be02/clang/test/ClangScanDeps/prebuilt-modules-in-stable-dirs.c#L2)). These tests used to require `SHELL`. Now they do not after [this](https://github.com/llvm/llvm-project/pull/156905/files#diff-a446c1da96800cde80387a7ce1b9cadf66639a70a7ff0c4f31bbf7884684816eR2) and [this](https://github.com/llvm/llvm-project/pull/156905/files#diff-1d266f055432e383e803ca92813fbe4001bf62cbd3ad800b4398d7b5d37cb48cR2), but we still have the limit of not supporting them on Windows. 

https://github.com/llvm/llvm-project/pull/168143