[mlir] Allow C-style escapes in Lexer

This patch passes the raw, unescaped value through to the rest of the stack. Partial escaping is a total pain to deal with, so we either need to implement escaping properly (ideally using a third party library like absl, I don't think LLVM has one that can handle the proper gamut of escape codes) or don't escape. I chose the latter for this patch.

PiperOrigin-RevId: 208608945
diff --git a/lib/Parser/Token.cpp b/lib/Parser/Token.cpp
index 9fc2b2f..2bce681 100644
--- a/lib/Parser/Token.cpp
+++ b/lib/Parser/Token.cpp
@@ -81,9 +81,8 @@
 /// Given a 'string' token, return its value, including removing the quote
 /// characters and unescaping the contents of the string.
 std::string Token::getStringValue() const {
-  // TODO: Handle escaping.
-
-  // Just drop the quotes off for now.
+  // Start by dropping the quotes.
+  // TODO: Un-escape the string here instead of passing through the raw content.
   return getSpelling().drop_front().drop_back().str();
 }