Support for affine integer sets

- introduce affine integer sets into the IR
- parse and print affine integer sets (both inline or outlined) similar to
  affine maps
- use integer set for IfStmt's conditional, and implement parsing of IfStmt's
  conditional

- fixed an affine expr paren omission bug while one this.

TODO: parse/represent/print MLValue operands to affine integer set references.
PiperOrigin-RevId: 207779408
diff --git a/lib/Parser/Lexer.h b/lib/Parser/Lexer.h
index f9cdfb6..51962fa 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 lexDoubleAtIdentifier(const char *tokStart);
   Token lexPrefixedIdentifier(const char *tokStart);
   Token lexNumber(const char *tokStart);
   Token lexString(const char *tokStart);