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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368121 91177308-0d34-0410-b5e6-96231b3b80d8