TableGen: Remove unused ParseForeachMode

Use the default ParseValueMode instead of ParseForeachMode when
parsing the rule

  ForeachDeclaration ::= ID '=' '[' ValueList ']'

because the only difference between the two is how an open brace '{'
is handled at the end. In the context of foreach, the 'in' keyword
will appear after the ForeachDeclaration, so this special handling
of '{' is not required.

Change-Id: I4d86bb73bab9ec26752e1273e5213df77cf28d1d
llvm-svn: 327119