[modules] Fix adding a templated friend functions to a namespace from another module.

[modules] Fix adding a templated friend functions to a namespace from another module.

When clang adds argument dependent lookup candidates, it can perform template
instantiation. For example, it can instantiate a templated friend function and
register it in the enclosing namespace's lookup table.

Fixes https://llvm.org/bugs/show_bug.cgi?id=24954

Reviewed by Richard Smith.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263634 91177308-0d34-0410-b5e6-96231b3b80d8