[TableGen] Emit a warning for unused template args

Add a warning to TableGen for unused template arguments in classes and
multiclasses, for example:

  multiclass Foo<int x> {
    def bar;
  }

  $ llvm-tblgen foo.td

  foo.td:1:20: warning: unused template argument: Foo::x
  multiclass Foo<int x> {
                     ^
A flag '--no-warn-on-unused-template-args' is added to disable the
warning. The warning is disabled for LLVM and sub-projects if
'LLVM_ENABLE_WARNINGS=OFF'.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D109359