[FIR] Set status.isOverride for fake overrides

Even though SO may not be correct
overrides sometimes, it feels more
natural to treat fake overrides as...
well, "overrides". And without it
we'd need to make the code in
`FirOverrideChecker` less intuitive.

analysis/analysis-api/testData/referenceResolve/arrayAccess/get.descriptors.txt

@@ -0,0 +1,2 @@
+Resolved to:
+0: (in kotlin.collections.MutableList) operator fun get(index: kotlin.Int): E

analysis/analysis-api/testData/referenceResolve/arrayAccess/get.txt

@@ -1,2 +1,2 @@
 Resolved to:
-0: (in kotlin.collections.MutableList) operator fun get(index: kotlin.Int): E
+0: (in kotlin.collections.MutableList) override operator fun get(index: kotlin.Int): E