Only complain about explicit instantiations following explicit

Only complain about explicit instantiations following explicit
specializations when the explicit instantiation was... explicitly
written, i.e., not the product of an explicit instantiation of an
enclosing class. Fixes this spurious warning when Clang builds LLVM:

/Volumes/Data/dgregor/Projects/llvm/lib/CodeGen/MachineDominators.cpp:22:1:
warning: explicit instantiation of 'addRoot' that occurs after an
explicit specialization will be ignored (C++0x extension) [-pedantic]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@100900 91177308-0d34-0410-b5e6-96231b3b80d8