Fix a super subtle bug: we were strlen'ing the entire buffer every time

we returned an lp_ keyword, which is N^2 work with a very large N
(particularly when your testcase is half a megabyte of .fir)!

This speeds up the parser from 12.75s -> 0.09s (!!!), which feels better
to me :)

lib/Dialect/FIRRTL/FIRToMLIR/FIRLexer.cpp

@@ -246,7 +246,7 @@ FIRToken FIRLexer::lexIdentifierOrKeyword(const char *tokStart) {
                               .Default(FIRToken::identifier);
     if (kind != FIRToken::identifier) {
       ++curPtr;
-      return FIRToken(kind, tokStart);
+      return formToken(kind, tokStart);
     }
   }