[clang-tidy] google-explicit-constructor: don't match in template instantiations

This helps avoiding false positives related to the recently added
std::initializer_list<> handling.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@222981 91177308-0d34-0410-b5e6-96231b3b80d8