CMake: build DLLs in the right directory and include them in the 'install' target (PR21719)

When CMake builds a dynamic library on Windows, the .dll file's location is
determined by the RUNTIME_OUTPUT_DIRECTORY, which we were previously not
setting.

This means for example that clang_rt.asan_dynamic-i386.dll will get built
and installed in the same directory as the corresponding .lib file,
instead of being built in the bin/ directory and not installed at all.

Differential Revision: http://reviews.llvm.org/D6508