[clang-tidy] Detect dependent initializer_lists in google-explicit-constructor.

Summary:
Detect constructors taking a single std::initializer_list even when it
is instantiation-dependent.

Reviewers: djasper

Reviewed By: djasper

Subscribers: curdeius, cfe-commits

Differential Revision: http://reviews.llvm.org/D7431

llvm-svn: 228289