cmake: Remove add_llvm_loadable_module()

Summary:
This function is very similar to add_llvm_library(),  so this patch merges it
into add_llvm_library() and replaces all calls to add_llvm_loadable_module(lib ...)
with add_llvm_library(lib MODULE ...)

Reviewers: philip.pfaffe, beanz, chandlerc

Reviewed By: philip.pfaffe

Subscribers: chapuni, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D51748

llvm-svn: 349839