Implement SE-0200 (extended escaping in string literals)

Supports string literals like #"foo"\n"bar"#.

test/Parse/raw_string_errors.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift
+
+#"\##("invalid")"#
+// expected-error@-1{{too many '#' characters in delimited escape}}
+// expected-error@-2{{invalid escape sequence in literal}}
+
+####"invalid"###
+// expected-error@-1{{unterminated string literal}}
+
+###"invalid"####
+// expected-error@-1{{too many '#' characters in closing delimiter}}
+// expected-error@-2{{consecutive statements on a line must be separated by ';'}}
+// expected-error@-3{{expected expression}}
+// expected-warning@-4{{string literal is unused}}