Fix non-deterministic iteration order when walking the specializations

of templates by using the newly introduce FoldingSetVector. This
preserves insertion order for all iteration of specializations.

I've also included a somewhat terrifying testcase that rapidly builds up
a large number of functions. This is enough that any system with ASLR
will have non-deterministic debug information generated for the test
case without the fix here as the debug information is generated in part
by walking these specializations.

llvm-svn: 156133