[ravi-pplx]

Slim LLVM distributions can fail ordinary C/C++ builds because absent auxiliary tools, such as llvm-cov, are included in internal-use-tools. Distributions using another standard library can also omit the bundled include/c++ directory, which is currently a required input.

Changes

• Skip wrappers and input labels for absent clang-format, clang-tidy, clangd, llvm-cov, llvm-dwp, and llvm-profdata.
• Make bundled libc++ headers optional while retaining the required Clang resource headers.
• Add regression coverage for full, slim, and partially populated distributions, including required-tool preservation and Darwin's libtool alias.
• Document the requirements for slim distributions.

Present optional tools remain available, and missing required tools still fail. Coverage and .dwp generation still require their corresponding tools; this change does not add support for those features when their tools are absent.

[helly25]

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