CodeGen: Don't set 'PMBuilder.DisableSimplifyLibCalls'

The simplify-libcalls pass has been removed from LLVM.  Thus
'PMBuilder.DisableSimplifyLibCalls' does not exist anymore.
The disabling/enabling of library call simplifications is
done through the TargetLibraryInfo which is already wired
up in Clang.

llvm-svn: 184458