Improve string concatentation & string templates code generation

Reuse StringBuilder instances for nested subexpressions.
(NB StringBuilder instance for string template with a string
concatenation inside an expression entry, such as `"${"a" + "b"}"`,
will not be reused, although that doesn't seem to be a real-life issue).

 #KT-18558 Fixed Target versions 1.1.4
 #KT-13682 Fixed Target versions 1.1.4

Join adjacent strings literals, escaped strings, and constant values
(in a language version that supports const val inlining).
Use StringBuilder#append(char) for single-character constants
(e.g., " " in "$a $b").

 #KT-17280 Fixed Target versions 1.1.4
 #KT-15235 Fixed Target versions 1.1.4

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/intrinsics/Concat.kt

@@ -48,8 +48,8 @@ class Concat : IntrinsicMethod() {
         if (element is KtBinaryExpression && element.operationReference.getReferencedNameElementType() == KtTokens.PLUS) {
             // LHS + RHS
             genStringBuilderConstructor(v)
-            codegen.invokeAppend(element.left)
-            codegen.invokeAppend(element.right)
+            codegen.invokeAppend(v, element.left)
+            codegen.invokeAppend(v, element.right)
         }
         else {
             // LHS?.plus(RHS)
@@ -57,7 +57,7 @@ class Concat : IntrinsicMethod() {
             genStringBuilderConstructor(v)
             v.swap()
             genInvokeAppendMethod(v, returnType)
-            codegen.invokeAppend(arguments.get(0))
+            codegen.invokeAppend(v, arguments.get(0))
         }
 
         v.invokevirtual("java/lang/StringBuilder", "toString", "()Ljava/lang/String;", false)