[Lexer] Add a couple of test cases for extended escaping str literal

Author: rintaro

test/Parse/raw_string.swift

@@ -132,3 +132,6 @@ _ = #"""
     ]
     """#
 // CHECK: "[\n    {\n        \"id\": \"12345\",\n        \"title\": \"A title that \\\"contains\\\" \\\\\\\"\"\n    }\n]"
+
+_ = #"# #"#
+// CHECK: "# #"

test/Parse/raw_string_errors.swift

@@ -16,3 +16,9 @@ let _ = ###"invalid"######
 // expected-error@-1{{too many '#' characters in closing delimiter}}{{24-27=}}
 // expected-error@-2{{consecutive statements on a line must be separated by ';'}}
 // expected-error@-3{{expected expression}}
+
+let _ = ##"""##
+  foobar
+  ##"""##
+// expected-error@-3{{multi-line string literal content must begin on a new line}}{{14-14=\n}}
+// expected-error@-2{{multi-line string literal closing delimiter must begin on a new line}}{{5-5=\n}}