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>

config_help.txt

@@ -82,7 +82,7 @@ Build options:
   -debug-and-release ... Build two versions of Qt, with and without
                          debugging turned on [yes] (Apple and Windows only)
   -optimize-debug ...... Enable debug-friendly optimizations in debug builds
-                         [auto] (Not supported with MSVC)
+                         [auto] (Not supported with MSVC or Clang toolchains)
   -optimize-size ....... Optimize release builds for size instead of speed [no]
   -optimized-tools ..... Build optimized host tools even in debug build [no]
   -force-debug-info .... Create symbol files for release builds [no]

configure.json

@@ -689,7 +689,7 @@
         },
         "optimize_debug": {
             "label": "Optimize debug build",
-            "condition": "!config.msvc && (features.debug || features.debug_and_release) && tests.optimize_debug",
+            "condition": "!config.msvc && !config.clang && (features.debug || features.debug_and_release) && tests.optimize_debug",
             "output": [ "privateConfig" ]
         },
         "optimize_size": {
@@ -1312,7 +1312,7 @@ Configure with '-qreal float' to create a build that is binary-compatible with 5
                 {
                     "type": "feature",
                     "args": "optimize_debug",
-                    "condition": "!config.msvc && (features.debug || features.debug_and_release)"
+                    "condition": "!config.msvc && !config.clang && (features.debug || features.debug_and_release)"
                 },
                 {
                     "type": "feature",