Allow augment decorator of uninstantiated templates with arguments

[tjprescott]

PR #1744 fixed an issue where one could target template instantiations with illogical results. The fix implemented in the PR implemented the following logic:

Error @@decorator(Foo<string>, "blah")
Error @@decorator(Foo<T>, "blah")
Okay @@decorator(Foo, "blah") // the template args left off

The problem with this is the following scenario:

@doc("Page of {T}", T)
model Page<T> {
  items: T[];
}

The equivalent augment of this would be
@@doc(Page<T>, "Page of {T}", T)

Unfortunately, due to #1744 this will yield an error.

The fix will likely require redesign of internals because it was discovered while implementing #1744 that somehow the TypeSpec cannot understand that T is different in this context than string.