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

[Maksim Levental via llvm-commits]

makslevental wrote:

> Right. lld headers should only be installed when `lld-headers` is present.

I just double checked on 918848d03bb9d0e06dea2ef588bda17ff961923c (i.e., prev commit) and this is the *current* behavior for lld (i.e., not introduced by this patch), which I guess is different from clang for some reason. E.g., I have 

```
-DLLVM_ENABLE_PROJECTS=mlir;lld
-DCMAKE_INSTALL_PREFIX=/Users/mlevental/dev_projects/llvm-project/llvm-install-release
-DLLVM_DISTRIBUTION_COMPONENTS=lld
```

and I get 

```
(iree) mlevental at maksims-MacBook-Pro cmake-build-release % ninja install install-distribution
....
(iree) mlevental at maksims-MacBook-Pro cmake-build-release % ls ../llvm-install-release/include 
lld     llvm    llvm-c  mlir    mlir-c
```


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