Rollup merge of #34383 - alexcrichton:less-compiler-checks, r=brson

Rollup merge of #34383 - alexcrichton:less-compiler-checks, r=brson

configure: Remove clang version checks

We no C++ and an incredibly small amount of C code as part of the build, so
there's not really much need for us to strictly check the version of compilers
as we're not really stressing anything. LLVM is a pretty huge chunk of C++ but
it should be the responsibility of LLVM to ensure that it can build with a
particular clang/gcc version, not ours (as this logic changes over time).

These version checks seem to basically just by us a regular stream of PRs every
six weeks or so when a new version is releases, so they're not really buying us
much. As a result, remove them and we can add then back piecemeal perhaps as a
blacklist if we really need to.