Append an executable suffix to the toolchain tool paths on Windows platform.

Append a default CMake suffix (CMAKE_EXECUTABLE_SUFFIX) for the build host to the toolchain tool path when adding a LLVM external project.
The tool binary files should contain .exe suffix to run them properly on Windows platform when building the projects.

Patch by Vlad Vereschaka.

Differential Revision: https://reviews.llvm.org/D65824

llvm-svn: 368121