Revert "Debug Info: Add basic support for external types references."

This reverts commit r242302. External type refs of this form were
never used by any LLVM frontend so this is effectively dead code.
(They were introduced to support clang module debug info, but in the
end we came up with a better design that doesn't use this feature at
all.)

rdar://problem/25897929

Differential Revision: https://reviews.llvm.org/D30917

llvm-svn: 297684