Remove Concat and StringPlus intrinsics in favor of IrStringConcatenation.

compiler/testData/codegen/box/strings/simpleStringPlus.kt

@@ -0,0 +1,18 @@
+fun f(s: String?, t: String): String {
+    return s.plus(t)
+}
+
+fun g(s: String, t: Any?): String {
+    return "$s$t"
+}
+
+fun h(s: String, t: Any?): String {
+    return s + t
+}
+
+fun box(): String {
+    if (f("O", "K") != "OK") return "Fail 1"
+    if (g("O", "K") != "OK") return "Fail 2"
+    if (h("O", "K") != "OK") return "Fail 3"
+    return "OK"
+}

compiler/testData/codegen/box/strings/singleConcatNullable.kt

@@ -0,0 +1,8 @@
+fun f(s: String?): String {
+    return "$s"
+}
+
+fun box(): String {
+    if (f(null) != "null") return "Fail"
+    return "OK"
+}

compiler/testData/codegen/box/strings/stringPlusOverride.kt

@@ -0,0 +1,9 @@
+operator fun String?.plus(p: String): String {
+    return "" + this
+}
+
+fun test(a: String?, b: String): String {
+    return a + b
+}
+
+fun box() = test("OK", " Fail")