[Support] Avoid UB in sys::fs::perms::operator~. NFC.

This was exposed in r297945 and r301220: the intermediate complement
is a 32-bit value, and casting it to 'perms' invokes UB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301373 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/FileSystem.h

@@ -116,7 +116,9 @@ inline perms &operator&=(perms &l, perms r) {
   return l;
 }
 inline perms operator~(perms x) {
-  return static_cast<perms>(~static_cast<unsigned short>(x));
+  // Avoid UB by explicitly truncating the (unsigned) ~ result.
+  return static_cast<perms>(
+      static_cast<unsigned short>(~static_cast<unsigned short>(x)));
 }
 
 class UniqueID {