[support] allocate exact size required for mapping in Support/Windws/Path.inc

Summary:
zturner suggested that mapped_file_region::init() on Windows seems to
create mappings that are larger than they need to be: Offset+Size
instead of Size. Indeed, that appears to be the case. I confirmed that
tests pass with mappings of just Size bytes, and fail with Size-1
bytes, suggesting that Size is indeed the correct value.

Reviewers: amccarth, zturner

Reviewed By: zturner

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D39876

llvm-svn: 317850