cmFortranParser: Simplify grammar by skipping unknown statements

Our Fortran grammar is a bare minimum to extract dependencies.
Other statement syntax can be ignored, so simply skip to the
end of unrecognized statements.  This allows some of our existing
productions to be dropped.

Source/cmFortranParser.y

@@ -102,14 +102,7 @@ static bool cmFortranParserIsKeyword(const char* word,
 
 code: /* empty */ | code stmt;
 
-stmt: keyword_stmt | assignment_stmt;
-
-assignment_stmt:
-  WORD ASSIGNMENT_OP other EOSTMT {
-    free($1);
-  }
-
-keyword_stmt:
+stmt:
   WORD EOSTMT {
     if (cmFortranParserIsKeyword($1, "interface")) {
       cmFortranParser* parser = cmFortran_yyget_extra(yyscanner);
@@ -212,12 +205,8 @@ keyword_stmt:
     cmFortranParser* parser = cmFortran_yyget_extra(yyscanner);
     cmFortranParser_RuleEndif(parser);
   }
-| WORD GARBAGE other EOSTMT {
-    free($1);
-  }
-| GARBAGE other EOSTMT
 | EOSTMT
-| error
+| error EOSTMT /* tolerate unknown statements until their end */
 ;