[clang module] Current Working Directory Pruning (#124786) When computing the context hash, `clang` always includes the compiler's working directory. This can lead to situations when the only difference between two compilations is the working directory, different module variants are generated. These variants are redundant. This PR implements an optimization that ignores the working directory when computing the context hash when safe. Specifically, `clang` checks if it is safe to ignore the working directory in `isSafeToIgnoreCWD`. The check involves going through compile command options to see if any paths specified are relative. The definition of relative path used here is that the input path is not empty, and `llvm::sys::path::is_absolute` is false. If all the paths examined are not relative, `clang` considers it safe to ignore the current working directory and does not consider the working directory when computing the context hash.
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