[JVM_IR] Don't use irPluginContext at all when compiling code fragment

This is an extended version of the fix in
57d912a5d4. We don't want to use any
plugins in code fragment compilation, but we still get them through
`project` because they are registered in IDEA. So we just fully
exclude `irPluginContext`.

#KT-63695 Fixed

compiler/tests-common-new/tests/org/jetbrains/kotlin/test/backend/ir/IrBackendInput.kt

@@ -115,7 +115,7 @@ sealed class IrBackendInput : ResultingArtifact.BackendInput<IrBackendInput>() {
             get() = backendInput.irModuleFragment
 
         override val irPluginContext: IrPluginContext
-            get() = backendInput.pluginContext
+            get() = backendInput.pluginContext!!
 
         override val diagnosticReporter: BaseDiagnosticsCollector
             get() = state.diagnosticReporter as BaseDiagnosticsCollector