[FIR] Introduce a feature flag for context-sensitive enum resolution

The feature was previously enabled unconditionally in K2 which
triggered a bug when an enum has an entry with the same name as itself.

#KT-58897 Fixed
#KT-52774

compiler/fir/analysis-tests/testData/resolve/unqualifiedEnum/nested.kt

@@ -1,3 +1,4 @@
+// LANGUAGE: +ContextSensitiveEnumResolutionInWhen
 enum class Outer {
     FIRST, SECOND;
 }