llvm-7.0 and clang-7.0 on SnowLeopard are working with minor adjustments

[Ken Cunningham]

With minor modifications to the patches and Portfile, both of these install and appear to be working fine. I'll set up a PR for the minor changes needed. -- Ken

$ port -v installed llvm-7.0
The following ports are currently installed:
  llvm-7.0 @7.0.0rc2_0 (active) platform='darwin 10' archs='x86_64' date='2018-09-03T23:29:34-0700'

$ port -v installed clang-7.0
The following ports are currently installed:
  clang-7.0 @7.0.0rc2_0+analyzer+libstdcxx (active) platform='darwin 10' archs='x86_64' date='2018-09-04T14:19:40-070