[modules] "Specialize" a function by actually specializing a function template
rather than by adding an overload and hoping that it's declared before the code
that calls it. (In a modules build, it isn't.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207133 91177308-0d34-0410-b5e6-96231b3b80d8