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/tests/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -1988,6 +1988,18 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("kt15235.kt")
+        public void testKt15235() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/stringOperations/kt15235.kt");
+            doTest(fileName);
+        }
+
+        @TestMetadata("nestedConcat.kt")
+        public void testNestedConcat() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/stringOperations/nestedConcat.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("nonNullableStringPlus.kt")
         public void testNonNullableStringPlus() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/stringOperations/nonNullableStringPlus.kt");