[lld] [lld] enable installing lld headers as part of distribution (PR #127123)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Description:

> `set(LLVM_DISTRIBUTION_COMPONENTS   lld   lld-cmake-exports)`
> and then ninja install-distribution one does not get an dist/include/lld folder.

I am not familiar with LLVM_DISTRIBUTION_COMPONENTS but this appears to be the intended behavior.
With `LLVM_DISTRIBUTION_COMPONENTS=clang;lld`, `ninja install-distribution` installs `bin/{clang,lld}` and a few symlinks. There is no header.

`LLVM_DISTRIBUTION_COMPONENTS=clang;clang-headers;lld` installs clang headers. This PR makes `lld-headers` installs lld headers.


https://github.com/llvm/llvm-project/pull/127123