[MSVC] Explicit specializations can be declared in any namespace (fix for http://llvm.org/PR13738)

MS compiler emits no errors in case of explicit specializations outside declaration enclosing namespaces, even when language extensions are disabled.
The patch is to suppress errors and emit extension warnings if explicit specializations are not declared in the corresponding namespaces.
This fixes PR13738.

Patch by Alexey Frolov.

Differential Revision: http://reviews.llvm.org/D8283

llvm-svn: 232800