QmakeProjectManager: Remove one use of ProcessArgs::ConstArgIterator

The code is effectively only called from the CocoPlugin and was
originally copied from QMakeStep::parserArgument() where it is
used to hide potentially syntactically wrong/unexpanded items
from the (anyway imprecise) internal .pro parser.

Using the expanded value is simpler and has about the same level of
incorrectness in a get/set roundtrip: Previously e.g. arguments
provided by macros were entirely dropped, now they can get replaced
by a fixed expansion. Both cases only affect complex setups of
the qmake step's "Additional arguments" case.

Change-Id: I9307371b5ca88415b0424077ceeaa4dc80f86afe
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/qmakeprojectmanager/qmakebuildconfiguration.cpp

@@ -418,16 +418,8 @@ QStringList QmakeBuildConfiguration::initialArgs() const
 {
     if (BuildStepList *buildSteps = this->buildSteps()) {
         if (auto qmakeStep = buildSteps->firstOfType<QmakeProjectManager::QMakeStep>()) {
-            QString arg = qmakeStep->userArguments.unexpandedArguments();
+            return ProcessArgs::splitArgs(qmakeStep->userArguments.arguments(),
-            ProcessArgs::ConstArgIterator it{arg};
+                                          project()->projectFilePath().osType());
-            QStringList result;
-
-            while (it.next()) {
-                if (it.isSimple())
-                    result << it.value();
-            }
-
-            return result;
         }
     }
     return {};