Move check for LLVM toolchain outside of repository_rule

[gferon]

Checking CC=clang in the implementation of the repository_rule doesn't work when using --incompatible_enable_cc_toolchain_resolution (for example when using https://github.com/grailbio/bazel-toolchain to bring a hermetic LLVM toolchain).

Moving the check for LLVM the repository_rule to the Swift toolchain impl used by all rules instead solves this issue.

I tested that the check (the failure message) is triggered on Linux with:

• only gcc installed
• a LLVM toolchain configured in WORKSPACE but without --incompatible_enable_cc_toolchain_resolution

Fixes #991

[keith]

nice thanks! note that with swift it's possible the hermetic llvm toolchain may not work since swift's llvm fork could differ in required ways, but this still seems like a nice fix