enable_language: Fix test for working compiler with CMP0126 NEW behavior

Update the logic that converts a try_compile result from a cache entry to a normal variable to tolerate an existing normal variable under CMP0126 NEW behavior. Otherwise the try_compile result is ignored because CMake uses the false value of the normal variable, and CMake incorrectly reports that the compiler does not work.

This went unnoticed for some languages (e.g. C and CXX) because the check for a working compiler is skipped if ABI detection works. It does affect other languages (e.g. CSharp).

Fixes: #22423 (closed)
Backport: release

changed milestone to %3.21.1

added triage:regression workflow:in-review labels

requested review from @marc.chevrier

assigned to @brad.king

Warnings:

• Eligible for the 3.21.x branch.

The warnings do not need to be fixed, but it is recommended to do so.

mentioned in issue #22423 (closed)

approved this merge request

Do: stage

added workflow:nightly-testing label and removed workflow:in-review label

resolved all threads

Do: merge

merged

mentioned in commit 095f49ec

mentioned in commit 8802d8f8

removed workflow:nightly-testing label

mentioned in issue #22451 (closed)

mentioned in merge request !6375 (merged)

resolved all threads