CompilerId/Features: Tolerate variables named for languages

If a `CMakeLists.txt` or `CMAKE_TOOLCHAIN_FILE` sets a variable named
`C`, `CXX`, or `CUDA`, we were previously comparing each enabled
language name to the value of that variable, rather than the name
itself.  Double-quote the string to take advantage of policy `CMP0054`,
but also add "x" prefixes to support projects that do not set the
policy.

Fixes: #22125