Our `codegen` test suite requires the LLVM `FileCheck` utility but unfortunately
this isn't always available in all custom LLVM roots (e.g. those specified via
`--llvm-root`). This commit adds a `./configure` option called
`--disable-codegen-tests` which will manually disable running these tests. In
the case that this option is passed we can forgo the need for the `FileCheck`
executable. Note that we still require `FileCheck` by default as we will attempt
to run these tests.
Closes#28667
a3fdde7453