Pin clang version on OSX in CI

[robdockins]

Recent OSX updates bumped the system clang version. Our tools are not quite ready to handle that version, and we are getting test failures in CI. This is a relatively regular occurrence, as OSX clang is relatively aggressive about using new LLVM versions. We should pin the version of clang used in our CI jobs somehow so we have control over this aspect.

[kquick]

Resolving via a combination of #618 and #624.

[atomb]

We could install the LLVM tools from binary tarballs from llvm.org instead of Xcode+Homebrew to prevent it from changing out from underneath us.

[kquick]

Documenting the general approach we've discussed:

• Github actions will use a single version
• Github actions will build the distribution tarball, for the version target
• The tests (via the tasty-sugar work in 618 and related) will support all "valid" LLVM versions (depending on LLVM); this will facilitate development inasmuch as it allows the use of whatever LLVM version is available to the developer instead of requiring a specific version.
• Our variance testing platform (fryingpan) will be used for testing a spectrum of LLVM versions.