KT-847 Allow "this" without {} in string templates

compiler/testData/diagnostics/tests/regressions/kt847.jet

@@ -0,0 +1,3 @@
+fun <T> T.mustBe(t : T) {
+    "$this must be$<!SYNTAX!>as<!>$t"
+}

compiler/testData/psi/StringTemplates.jet

@@ -7,6 +7,7 @@ fun demo() {
   fun buzz(f : fun () : Any?) : Int  = 1
   val sdf = 1
   val foo = 3;
+    "$this must be$as$t"
     "$abc"
     "$abc\"
     "$"

compiler/testData/psi/StringTemplates.txt

@@ -157,6 +157,25 @@ JetFile: StringTemplates.jet
             PsiElement(INTEGER_LITERAL)('3')
         PsiElement(SEMICOLON)(';')
         PsiWhiteSpace('\n    ')
+        STRING_TEMPLATE
+          PsiElement(OPEN_QUOTE)('"')
+          SHORT_STRING_TEMPLATE_ENTRY
+            PsiElement(SHORT_TEMPLATE_ENTRY_START)('$')
+            THIS_EXPRESSION
+              REFERENCE_EXPRESSION
+                PsiElement(this)('this')
+          LITERAL_STRING_TEMPLATE_ENTRY
+            PsiElement(REGULAR_STRING_PART)(' must be')
+          SHORT_STRING_TEMPLATE_ENTRY
+            PsiElement(SHORT_TEMPLATE_ENTRY_START)('$')
+            PsiErrorElement:Keyword cannot be used as a reference
+              PsiElement(as)('as')
+          SHORT_STRING_TEMPLATE_ENTRY
+            PsiElement(SHORT_TEMPLATE_ENTRY_START)('$')
+            REFERENCE_EXPRESSION
+              PsiElement(IDENTIFIER)('t')
+          PsiElement(CLOSING_QUOTE)('"')
+        PsiWhiteSpace('\n    ')
         STRING_TEMPLATE
           PsiElement(OPEN_QUOTE)('"')
           SHORT_STRING_TEMPLATE_ENTRY