This patch allows `lld-headers` and `lld-libraries` in
`LLVM_DISTRIBUTION_COMPONENTS` to be specified and thus enable piecewise
installation of `lld/**/*.h` headers and/or lld libraries (both in
shared and static builds).
This is similar to use cases such as
`clang;clang-headers;clang-libraries`. Note when `lld-libraries` is
present, `llvm-libraries` must be present as well because various lld
libraries depend on various llvm libraries.
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