Add some scaffolding for parsing affine maps:
- parsing affine map identifiers
- place-holder classes for AffineMap
- module contains a list of affine maps (defined at the top level).

PiperOrigin-RevId: 202336919
diff --git a/lib/Parser/Lexer.h b/lib/Parser/Lexer.h
index 139dfa6..0301a35 100644
--- a/lib/Parser/Lexer.h
+++ b/lib/Parser/Lexer.h
@@ -60,6 +60,7 @@
   Token lexComment();
   Token lexBareIdentifierOrKeyword(const char *tokStart);
   Token lexAtIdentifier(const char *tokStart);
+  Token lexAffineMapId(const char *tokStart);
   Token lexNumber(const char *tokStart);
 };