Simplify Configure in two ways

• No need to distinguish between gcc-llvm and clang. First of all, gcc-llvm is quite old and surely unmaintained by now. Second of all, none of the code actually care about that distinction!

  Now, it does make sense to consider C multiple frontends for LLVMs in the form of clang vs clang-cl (same clang, yes, but tweaked interface). But this is better handled in terms of "gccish vs mvscish" and "is LLVM", yielding 4 combinations. Therefore, I don't think it is useful saving the existing code for that.

• Get the remaining CC_LLVM_BACKEND, and also TABLES_NEXT_TO_CODE in mk/config.h the normal way, rather than hacking it post-hoc. No point keeping these special cases around for now reason.

Oh, and as part of the above I also renamed some things where it felt appropriate.

• GccIsClang -> CcLlvmBackend (was never about gcc-llvm, or Apple's renamed clang, to be clear).

• llvm_CC_FLAVOR -> CC_LLVM_BACKEND

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