Avoid the gcc warning -Wno-non-template-friend

To avoid this warning, friend declarations inside template classes
must be declared as templates:
http://stackoverflow.com/questions/4039817/friend-declaration-declares-a-non-template-function



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@199505 91177308-0d34-0410-b5e6-96231b3b80d8