It turns out that linkers (at least, the Darwin linker) don't necessarily

It turns out that linkers (at least, the Darwin linker) don't necessarily
do the right thing with mixed-visibility symbols, so disable the visibility
optimization where that's possible, i.e. with template classes (since it's
possible that an arbitrary template might be subject to an explicit
instantiation elsewhere).  447.dealII actually does this.

I've put the code under an option that's currently not hooked up to anything.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110374 91177308-0d34-0410-b5e6-96231b3b80d8