IR: clear callToSubstitutedDescriptorMap for each file to avoid leaking memory

This map is populated during psi2ir and is only used in
insertImplicitCasts, after which it makes sense to clear it, otherwise
it strongly references a lot of descriptors for calls in other files
which would never be used.

compiler/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/generators/GeneratorContext.kt

@@ -32,7 +32,7 @@ class GeneratorContext(
     val constantValueGenerator: ConstantValueGenerator,
     override val irBuiltIns: IrBuiltIns
 ) : IrGeneratorContext {
-    val callToSubstitutedDescriptorMap = mutableMapOf<IrDeclarationReference, CallableDescriptor>()
+    internal val callToSubstitutedDescriptorMap = mutableMapOf<IrDeclarationReference, CallableDescriptor>()
 
     val sourceManager = PsiSourceManager()
 

compiler/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/generators/ModuleGenerator.kt

@@ -90,7 +90,10 @@ class ModuleGenerator(
         }
 
         irFile.acceptChildrenVoid(IrSyntheticDeclarationGenerator(context))
+
         insertImplicitCasts(irFile, context)
+        context.callToSubstitutedDescriptorMap.clear()
+
         irFile.acceptVoid(AnnotationGenerator(context))
 
         irFile.patchDeclarationParents()