gnu: LLVM: Promote LLVM 9 to the default 'llvm' package.

gnu: LLVM: Promote LLVM 9 to the default 'llvm' package.

* gnu/packages/llvm.scm (llvm-8): Rename to ...
(llvm): ... this.  Update to 9.0.1.
[license]: Change to ASL2.0.
(clang-runtime-from-llvm)[license]: Inherit from LLVM.
(clang-from-llvm)[license]: Conditionally choose license based on version.
(llvm-9, clang-runtime-8, clang-8, clang-toolchain-8): New public variables.
(llvm-7): Inherit from LLVM-8 (for the license change).
(llvm-6): Inherit from LLVM-7.
(llvm-3.9.1): Inherit from LLVM-6.
(llvm-3.8): Inherit from LLVM-3.9.1.
(llvm-3.7): Inherit from LLVM-3.8.
(llvm-3.6): Inherit from LLVM-3.7.
(llvm-3.5): Inherit from LLVM-3.6.
(libcxx, libclc, libomp): Update to 9.0.1.