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

[inglorion]

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.

[zturner]

yay, huge space gains ftw.

llvm/lib/Support/Windows/Path.inc

std::error_code mapped_file_region::init(int FD, uint64_t Offset,
   switch (Mode) {
   case readonly:  flprotect = PAGE_READONLY; break;
   case readwrite: flprotect = PAGE_READWRITE; break;
   case priv:      flprotect = PAGE_WRITECOPY; break;
   }
 
   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());
     return ec;
   }
 
   DWORD dwDesiredAccess;
   switch (Mode) {