[Syntax] add test for escaped identifier

Author: omochi

test/Syntax/tokens_escaped_identifier.swift

@@ -0,0 +1,21 @@
+// RUN: %swift-syntax-test -input-source-filename %s -dump-full-tokens | %FileCheck %s
+let /*leading trivia*/ `if` = 3
+print(/*leading trivia*/ `if` )
+
+// CHECK-LABEL: 2:25
+// CHECK-NEXT:(token identifier
+// CHECK-NEXT: (trivia block_comment/*leading trivia*/)
+// CHECK-NEXT: (trivia space 1)
+// CHECK-NEXT: (trivia backtick 1)
+// CHECK-NEXT: (text="if")
+// CHECK-NEXT: (trivia backtick 1)
+// CHECK-NEXT: (trivia space 1))
+
+// CHECK-LABEL: 3:27
+// CHECK-NEXT:(token identifier
+// CHECK-NEXT: (trivia block_comment/*leading trivia*/)
+// CHECK-NEXT: (trivia space 1)
+// CHECK-NEXT: (trivia backtick 1)
+// CHECK-NEXT: (text="if")
+// CHECK-NEXT: (trivia backtick 1)
+// CHECK-NEXT: (trivia space 1))