[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

llvm/lib/Support/Windows/Path.inc

@@ -734,8 +734,8 @@ std::error_code mapped_file_region::init(int FD, uint64_t Offset,
 
   HANDLE FileMappingHandle =
       ::CreateFileMappingW(FileHandle, 0, flprotect,
-                           (Offset + Size) >> 32,
-                           (Offset + Size) & 0xffffffff,
+                           Size >> 32,
+                           Size & 0xffffffff,
                            0);
   if (FileMappingHandle == NULL) {
     std::error_code ec = mapWindowsError(GetLastError());