[dsymutil] Don't mark forward declarations as canonical.

This patch completes the work done by Frederic Riss to addresses
dsymutil incorrectly considering forward declaration as canonical during
uniquing. This resulted in references to the forward declaration even
after the definition was encountered.

In addition to the test provided by Alexander Shaposhnikov in D29609, I
added another test to cover several scenarios that were mentioned in his
conversation with Fred. We now also check that uniquing still occurs
after the definition was encountered.

For more context please refer to D29609

Differential revision: https://reviews.llvm.org/D37127

llvm-svn: 312274