Merging rr280190:

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r280190 | rsmith | 2016-08-30 20:15:21 -0600 (Tue, 30 Aug 2016) | 12 lines

PR12298 et al: don't recursively instantiate a template specialization from
within the instantiation of that same specialization. This could previously
happen for eagerly-instantiated function templates, variable templates,
exception specifications, default arguments, and a handful of other cases.

We still have an issue here for default template arguments that recursively
make use of themselves and likewise for substitution into the type of a
non-type template parameter, but in those cases we're producing a different
entity each time, so they should instead be caught by the instantiation depth
limit. However, currently we will typically run out of stack before we reach
it. :(

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_39@282636 91177308-0d34-0410-b5e6-96231b3b80d8