llvm dev - Aug 2020 - TableGen enhancements

I puttered a bit with the error handling in TableGen. The Searchable Tables
backend currently produces the following error message when a generic enum
specifies an unknown FilterClass (an error that the parser cannot detect):

search-test.td:3:1: error: Enum FilterClass 'BEntryX' does not exist
def BValues : GenericEnum {
^

Now it produces this message:

search-test.td:4:21: error: Enum FilterClass 'BEntryX' does not exist
  let FilterClass = "BEntryX";
                             ^  [points to the open quote]

That was not a particularly difficult accomplishment. The horror comes when
facing all the error message calls throughout the backends.