Flatten nested string concatenation expressions into a single IrStringConcatenation, in a separate lowering phase.

Consolidating these into IrStringConcatenations allows the backend to produce efficient code for string concatenations (e.g., using StringBuilder for JVM).

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

@@ -8,14 +8,34 @@ fun test1(s1: String, s2: String, s3: String) =
 fun test2(s1: String, s2: String, s3: String) =
         s1 + (s2 + s3)
 
-fun test3(s1: String, s2: String, s3: String) =
+fun test3(s1: String, s2: String, s3: String, s4: String) =
+        ((s1 + s2) + ((s3 + s4)))
+
+fun test4(s1: String, s2: String, s3: String) =
         "s1: $s1; " +
         "s2: $s2; " +
         "s3: $s3"
 
+fun test5(s1: String, s2: String, s3: String) =
+        "${"s1:" + "${" " + s1};"} " +
+        "${"s2:" + "${" " + s2};"} " +
+        "${"s3:" + "${" " + s3}"}"
+
+fun test6(s1: String, s2: String, s3: String) =
+    "${"s1:" + "${" " + s1};"} ${"s2:" + "${" " + s2};"} ${"s3:" + "${" " + s3}"}"
+
+fun test7(s1: String, s2: String, s3: String): String {
+    fun foo(s: String) = s
+    return "foo: " + foo(s1 + s2 + " ${foo("\${s3.length} = ${s3.length}")}")
+}
+
 fun box(): String {
     assertEquals("123", test1("1", "2", "3"))
     assertEquals("123", test2("1", "2", "3"))
-    assertEquals("s1: 1; s2: 2; s3: 3", test3("1", "2", "3"))
+    assertEquals("1234", test3("1", "2", "3", "4"))
+    assertEquals("s1: 1; s2: 2; s3: 3", test4("1", "2", "3"))
+    assertEquals("s1: 1; s2: 2; s3: 3", test5("1", "2", "3"))
+    assertEquals("s1: 1; s2: 2; s3: 3", test6("1", "2", "3"))
+    assertEquals("foo: 12 \${s3.length} = 1", test7("1", "2", "3"))
     return "OK"
 }