Sketch out parser/IR support for OperationInst, and a new Instruction base
class.

Introduce an Identifier class to MLIRContext to represent uniqued identifiers,
introduce string literal support to the lexer, introducing parser and printer
support etc.

PiperOrigin-RevId: 202592007
diff --git a/lib/Parser/Lexer.h b/lib/Parser/Lexer.h
index 0301a35..f0274fe 100644
--- a/lib/Parser/Lexer.h
+++ b/lib/Parser/Lexer.h
@@ -62,6 +62,7 @@
   Token lexAtIdentifier(const char *tokStart);
   Token lexAffineMapId(const char *tokStart);
   Token lexNumber(const char *tokStart);
+  Token lexString(const char *tokStart);
 };
 
 } // end namespace mlir