Disable -optimize-debug for Clang

It results in Xcode outputting a warning when debugging:

  [app name] was compiled with optimization - stepping may behave oddly;
  variables may not be available.

And the warning is correct, debugging is broken in this situation.

Likely caused by Clang treating -Og as -O1:

  https://reviews.llvm.org/D24998



Change-Id: I25d6bf1e65c81cc5be92b9847f7d5dd6754a0177
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>