Add all patches from base llvm/clang/lld/lldb 6.0 to devel/llvm60

This adds all the patches that were applied in the past to head, under
contrib/llvm. After these, there only minimal diffs left between the
port sources and the base sources.

Most of these remaining diffs are due to #ifdef shortcuts in the base
sources, because we don't compile certain features in. Other diffs are
because the port has applied a few changes that we don't have in base.

While here, use Makefile.LICENSE from the devel/llvm-devel port.

Approved by: brooks (maintainer)
Reviewed by: brooks
PR: 212343, 225128, 225471, 226388, 226658, 226872, 229050, 230444, 230604, 231355
MFH: 2018Q4
Differential Revision: https://reviews.freebsd.org/D17702