[SR-197] Skip generic specialization when generic is <<error type>>

[EliaCereda]

This disables the 'cannot specialize non-generic type' error message when the type is <<error type>> since it doesn't convey any additional information to the user.

This code represents an instance of this problem:

$ cat a.swift 
typealias SomeType = NonExistentType
func foo(bar: SomeType<Int>) {

}
$ swiftc -c  a.swift 
a.swift:1:22: error: use of undeclared type 'NonExistentType'
typealias SomeType = NonExistentType
                     ^~~~~~~~~~~~~~~
a.swift:2:15: error: cannot specialize non-generic type '<<error type>>'
func foo(bar: SomeType<Int>) {

This would be the first time I contribute code to Swift, so I might have made some rookie mistake. In particular, from what I saw on the discussion of #432, this might not be the place where the problem should be addressed.

I'm currently running the test suite, but I think it should pass since I haven't found any test checking this particular case. Should one be added?

EDIT: the tests just finished:

  Expected Passes    : 2350
  Expected Failures  : 5
  Unsupported Tests  : 34

[lattner]

Sorry for the delay reviewing this. The patch looks great to me. Please add a test case to the test suite though.

[EliaCereda]

This seems to cover the issue. I think that's the right place where this should be tested, since that's the part that tests typealias SomeType = NonExistentType

[lattner]

Looks great, thanks. Please close SR-197 when you get a chance,

-Chris