[LL FIR] FileStructure: resolve FirFile to body phase as all other elements

This is the correct behavior as we have to resolve to body phase
to build CFG and run diagnostics collector correctly

^KT-65780 Fixed

analysis/low-level-api-fir/testData/getOrBuildFir/inImport/middleImportNamePart.txt

@@ -6,4 +6,4 @@ FIR element rendered:
 import a.b.c
 
 FIR FILE:
-FILE: [ResolvedTo(IMPORTS)] middleImportNamePart.kt
+FILE: [ResolvedTo(BODY_RESOLVE)] middleImportNamePart.kt