Support: mapped_file_region: Pass MAP_NORESERVE to mmap

This allows mapping larger files, delaying OOM failures until too many
pages of them are accessed.  This is makes the behavior of the
mapped_file_region in this regard consistent between its "Unix" and
"Windows" implementations.

Guard the code witih #if defined(MAP_NORESERVE), consistent with other
uses of MAP_NORESERVE in llvm-project, because some FreeBSD versions do
not provide this flag.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D96626

llvm/lib/Support/Unix/Path.inc

@@ -815,6 +815,9 @@ std::error_code mapped_file_region::init(int FD, uint64_t Offset,
 
   int flags = (Mode == readwrite) ? MAP_SHARED : MAP_PRIVATE;
   int prot = (Mode == readonly) ? PROT_READ : (PROT_READ | PROT_WRITE);
+#if defined(MAP_NORESERVE)
+  flags |= MAP_NORESERVE;
+#endif
 #if defined(__APPLE__)
   //----------------------------------------------------------------------
   // Newer versions of MacOSX have a flag that will allow us to read from