clang-cl: Support building DLLs (PR17083)

This adds driver support for building DLLs (the /LD and /LDd flags).
It basically does two things: runtime selection and passing -dll and
-implib to the linker.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190428 91177308-0d34-0410-b5e6-96231b3b80d8