Fix: Always honor LLVM_LIBDIR_SUFFIX.

Static libraries where installed into "lib${LLVM_LIBDIR_SUFFIX}" while
shared ones into "lib". I found no justification for this behaviour.
This patch changes both types of libraries to be install into
"lib${LLVM_LIBDIR_SUFFIX}". LLVM and clang use the same behaviour.

This fixes llvm.org/PR27305.

llvm-svn: 265872