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/bytecodeText/stringOperations/concat.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A() {
 
     override fun toString(): String {

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

@@ -1,16 +1,41 @@
-// IGNORE_BACKEND: JVM_IR
+// Uses 1 StringBuilder
 fun test1(s1: String, s2: String, s3: String) =
         (s1 + s2) + s3
 
+// Uses 1 StringBuilder
 fun test2(s1: String, s2: String, s3: String) =
         s1 + (s2 + s3)
 
+// Uses 1 StringBuilder
 fun test3(s1: String, s2: String, s3: String, s4: String) =
         ((s1 + s2) + ((s3 + s4)))
 
+// Combination of String.plus and string literal
+// Uses 1 StringBuilder
 fun test4(s1: String, s2: String, s3: String) =
         "s1: $s1; " +
         "s2: $s2; " +
         "s3: $s3"
 
-// 4 NEW java/lang/StringBuilder
+// Combination of String.plus and nested string literal
+// Uses 1 StringBuilder
+fun test5(s1: String, s2: String, s3: String) =
+        "${"s1:" + "${" " + s1};"}; " +
+        "${"s2:" + "${" " + s2};"}; " +
+        "${"s3:" + "${" " + s3}"}"
+
+// Top-level string concatenation element is a string literal
+// Uses 1 StringBuilder
+fun test6(s1: String, s2: String, s3: String) =
+    "${"s1:" + "${" " + s1};"} ${"s2:" + "${" " + s2};"} ${"s3:" + "${" " + s3}"}"
+
+// Uses 3 StringBuilders:
+// - In return expression
+// - In argument to 1st call to foo()
+// - In argument to 2nd call to foo() inside string literal
+fun test7(s1: String, s2: String, s3: String): String {
+    fun foo(s: String) = s
+    return "foo: " + foo(s1 + s2 + " ${foo("\${s3.length} = ${s3.length}")}")
+}
+
+// 9 NEW java/lang/StringBuilder

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A() {
 
     override fun toString(): String {

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun test(b: Byte, s: Short, i: Int, l: Long) {
     "$b"
     "$s"