Add FIR enhancement tests, fix some exceptions / problems around them

Test data and tests themselves are based on
compiler/testData/loadJava/compiledJava

compiler/testData/loadJava/compiledJava/notNull/NotNullField.fir.txt

@@ -0,0 +1,2 @@
+public open class NotNullField : R|java/lang/Object| {
+}

compiler/testData/loadJava/compiledJava/notNull/NotNullIntArray.fir.txt

@@ -0,0 +1,4 @@
+public open class NotNullIntArray : R|java/lang/Object| {
+    @R|org/jetbrains/annotations/NotNull|() public open operator function hi(): R|kotlin/IntArray|
+
+}

compiler/testData/loadJava/compiledJava/notNull/NotNullMethod.fir.txt

@@ -0,0 +1,4 @@
+public open class NotNullMethod : R|java/lang/Object| {
+    @R|org/jetbrains/annotations/NotNull|() public open operator function hi(): R|ft<java/lang/String, java/lang/String>|
+
+}

compiler/testData/loadJava/compiledJava/notNull/NotNullObjectArray.fir.txt

@@ -0,0 +1,4 @@
+public open class NotNullObjectArray : R|java/lang/Object| {
+    @R|org/jetbrains/annotations/NotNull|() public open operator function hi(): R|kotlin/Array<ft<java/lang/Object, java/lang/Object?>>|
+
+}

compiler/testData/loadJava/compiledJava/notNull/NotNullParameter.fir.txt

@@ -0,0 +1,4 @@
+public open class NotNullParameter : R|java/lang/Object| {
+    public open operator function hi(@R|org/jetbrains/annotations/NotNull|() param: R|ft<java/lang/String, java/lang/String>|): R|kotlin/Unit|
+
+}