Unresolved symbol linker errors building swift-llvm in Release mode

Hi all,

I've been testing out the Windows build in release mode.

An exception is thrown from the MSVC standard library compiling the swift
stdlib in release mode instead of debug mode using clang-cl or MSVC, so I
wanted to see what's going on.

To get swift.exe to link in release mode, I also need to build LLVM in
release mode.

I've tried doing this, but get several linker errors linking every single
executable in the project:

Examples are:

`llvm::Value::dump()`
`SubMultiClassReference::dump()`

and a couple more.

Some of these examples have changed in upstream LLVM, but the changes are
too complicated to pull into swift-llvm unfortunately.

I was surprised - has anyone tried building swift-llvm's swift-4.0-branch
in Release mode? Isn't this necessary to build Swift in release mode?

Thanks,
Hugh