[mlir][OpAsmParser] Add parseString method

[zero9178]

Summary

Basically every kind of parseOptional* method in DialectAsmParser has a corresponding parse* method which will emit an error if the requested token has not been found. An odd one out of this rule is parseOptionalString which does not have a corresponding parseString method.

This patch adds that method and implements it in basically the same fashion as parseKeyword, by first going through parseOptionalString and emitting an error on failure.

[lattner]

LGTM, thanks!

mlir/include/mlir/IR/DialectImplementation.h

public:
   virtual ParseResult parseOptionalComma() = 0;
 
   /// Parse a `=` token.
   virtual ParseResult parseEqual() = 0;
 
   /// Parse a `=` token if present.
   virtual ParseResult parseOptionalEqual() = 0;
 
+  /// Parse a quoted string token.
+  ParseResult parseString(StringRef *string) {
+    auto loc = getCurrentLocation();
+    if (parseOptionalString(string))
+      return emitError(loc, "expected string");
+    return success();
+  }
+
   /// Parse a quoted string token if present.
   virtual ParseResult parseOptionalString(StringRef *string) = 0;
 
   /// Parse a given keyword.
   ParseResult parseKeyword(StringRef keyword, const Twine &msg = "") {
     auto loc = getCurrentLocation();
     if (parseOptionalKeyword(keyword))
       return emitError(loc, "expected '") << keyword << "'" << msg;