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/testData/codegen/box/constants/kt9532.kt

@@ -1,5 +1,4 @@
 // TODO: muted automatically, investigate should it be ran for JS or not
-// IGNORE_BACKEND: JS
 
 object A {
     const val a: String = "$"
@@ -7,7 +6,7 @@ object A {
     const val c = 10000
 
     val bNonConst = "1234$a"
-    val bNullable: String = "1234$a"
+    val bNullable: String? = "1234$a"
 }
 
 object B {
@@ -16,7 +15,7 @@ object B {
     const val c = 10000
 
     val bNonConst = "1234$a"
-    val bNullable: String = "1234$a"
+    val bNullable: String? = "1234$a"
 }
 
 fun box(): String {
@@ -26,8 +25,8 @@ fun box(): String {
 
     if (A.c !== B.c) return "Fail 3: A.c !== B.c"
 
-    if (A.bNonConst === B.bNonConst) return "Fail 5: A.bNonConst == B.bNonConst"
-    if (A.bNullable === B.bNullable) return "Fail 6: A.bNullable == B.bNullable"
+    if (A.bNonConst !== B.bNonConst) return "Fail 4: A.bNonConst !== B.bNonConst"
+    if (A.bNullable !== B.bNullable) return "Fail 5: A.bNullable !== B.bNullable"
 
     return "OK"
 }

compiler/testData/codegen/box/constants/kt9532_lv10.kt

@@ -0,0 +1,34 @@
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+// LANGUAGE_VERSION: 1.0
+
+object A {
+    const val a: String = "$"
+    const val b = "1234$a"
+    const val c = 10000
+
+    val bNonConst = "1234$a"
+    val bNullable: String? = "1234$a"
+}
+
+object B {
+    const val a: String = "$"
+    const val b = "1234$a"
+    const val c = 10000
+
+    val bNonConst = "1234$a"
+    val bNullable: String? = "1234$a"
+}
+
+fun box(): String {
+    if (A.a !== B.a) return "Fail 1: A.a !== B.a"
+
+    if (A.b !== B.b) return "Fail 2: A.b !== B.b"
+
+    if (A.c !== B.c) return "Fail 3: A.c !== B.c"
+
+    if (A.bNonConst === B.bNonConst) return "Fail 4: A.bNonConst === B.bNonConst"
+    if (A.bNullable === B.bNullable) return "Fail 5: A.bNullable === B.bNullable"
+
+    return "OK"
+}