Package/flang: Updated to use own version of llvm (#8766)

Flang now uses its own version of llvm and clang (called flang-driver). This is
handled by adding flang-specific versions of the LLVM package and updates flang
to depend on those versions.

var/spack/repos/builtin/packages/pgmath/package.py

@@ -30,9 +30,11 @@ class Pgmath(CMakePackage):
     """Flang's math library"""
 
     homepage = "https://github.com/flang-compiler/flang"
+    url      = "https://github.com/flang-compiler/flang/archive/flang_20180612.tar.gz"
     git      = "https://github.com/flang-compiler/flang.git"
 
     version('develop', branch='master')
+    version('20180612', '62284e26214eaaff261a922c67f6878c')
 
     conflicts("%gcc@:7.1.9999")