Merging r369834:

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r369834 | rsmith | 2019-08-24 04:30:00 +0200 (Sat, 24 Aug 2019) | 8 lines

PR42513: Enter the proper DeclContext before substituting into an
default template argument expression.

We already did this for type template parameters and template template
parameters, but apparently forgot to do so for non-type template
parameters. This causes the substituted default argument expression to
be substituted in the proper context, and in particular to properly mark
its subexpressions as odr-used.
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llvm-svn: 370038