RFC - Adding support for renaming clang via a CMake macro

I have just posted a draft patch (https://reviews.llvm.org/D103125) that allows a user to rename the clang executable via a cmake macro.

This is presumably useful for vendors that provide a proprietary clang/llvm-based compiler as well as the upstream clang on their platforms. Having the compilers use different names makes it easier for users to disambiguate which is which.

Of course, this can be accomplished by simply renaming the binaries/symlinks after the build, but that has the drawback of tests, utilities, etc. not knowing about the new name. Implementing this as a cmake configuration allows for a standard way of doing this that can be tested using upstream buildbots to prevent downstream breaks due to upstream changes.

Furthermore, since the llvm-config utility knows the name/path of the clang driver executable, build tools can easily learn to find anything that is named based on the name of the clang driver (libraries, tools, etc.).

Please let me know what you think about this proposal either by responding to this RFC or providing your feedback on the associated Phabricator review.