gnu: LLVM, Clang: Update to 7.0.1.

gnu: LLVM, Clang: Update to 7.0.1.

* gnu/packages/patches/clang-7.0-libc-search-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/llvm.scm (llvm, clang-runtime, clang): Update to 7.0.1.
(llvm-7.0.1): Remove variable.
(clang-from-llvm)[arguments]: Adjust phase to match Clang 7.
(llvm-6, clang-runtime-6, clang-6): New public variables.
* gnu/packages/dlang.scm (ldc)[native-inputs]: Change LLVM and CLANG to
LLVM-6.0 and CLANG-6.0.
* gnu/packages/gl.scm (mesa)[inputs]: Change from LLVM to LLVM-6.