Set the folder for libraries to 'lld libraries'. NFC.

In a UI such as XCode, LLVM source files are in 'libraries' while clang
files are in 'clang libraries'.

This change moves the lld source to 'lld libraries' to make code browsing easier.

It should be NFC as the build itself is still the same, just the structure in a
UI differs.

llvm-svn: 257001