Refactor argument serialization logic when executing process. NFC.

This patch refactors the argument serialization logic used in the Execute
function, used to launch new Windows processes. There is a critical step that
joins char** arguments into a single string, building the command line used to
launch the new process, and the readability of this code is improved if this
part is refactored in its own helper function.

Patch by Rafael Auler!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216411 91177308-0d34-0410-b5e6-96231b3b80d8