Rename sys::Process::GetArgumentVector -> sys::windows::GetCommandLineArguments

GetArgumentVector (or GetCommandLineArguments) is very Windows-specific.
I think it doesn't make much sense to provide that function from sys::Process.

I also made a change so that the function takes a BumpPtrAllocator
instead of a SpecificBumpPtrAllocator. The latter is the class to call
dtors, but since char * is trivially destructible, we should use the
former class.

Differential Revision: https://reviews.llvm.org/D45641

llvm-svn: 330216