IR: Don't use IrStringConcatenation for ordinary toString calls

We can only use IrStrinConcatentation to represent calls to Any?.toString
and toString calls on primitive types. Otherwise, x.toString() and "$x"
are observably different when x is a non-null type with null value
(e.g., an @NotNull value coming from Java).

compiler/testData/codegen/bytecodeText/stringOperations/stringBuilderToString.kt

@@ -0,0 +1,12 @@
+val chars = listOf('O', 'K')
+
+fun box(): String {
+    val b = StringBuilder()
+    for (c in chars) {
+        b.append(c)
+    }
+    return b.toString()
+}
+
+// 0 INVOKESTATIC java/lang/String.valueOf
+// 1 INVOKEVIRTUAL java/lang/StringBuilder.toString