QtSupport: Fix handling of not yet saved *.ui files

Since

  commit fb3e18bd4b
  Fix missing header guards in UI files

the uic input comes from a file (on disk) and thus the source contents
from the edited and not yet saved *.ui file (Designer mode) was ignored.

Revert the relevant part. This should not introduce a regression as

  commit 47312b04c0
  QtSupport: Fix header guard for ui files

basically superseded the first change, handling the proper inclusion of
the generated header file.

Fixes: QTCREATORBUG-22740
Change-Id: Iec6550817941eea99f823c377680b9a6fa5564e8
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qtsupport/uicgenerator.cpp

@@ -67,7 +67,7 @@ Utils::FilePath UicGenerator::command() const
 
 QStringList UicGenerator::arguments() const
 {
-    return {"-p", source().toString()};
+    return {"-p"};
 }
 
 FileNameToContentsHash UicGenerator::handleProcessFinished(QProcess *process)
@@ -87,6 +87,12 @@ FileNameToContentsHash UicGenerator::handleProcessFinished(QProcess *process)
     return result;
 }
 
+void UicGenerator::handleProcessStarted(QProcess *process, const QByteArray &sourceContents)
+{
+    process->write(sourceContents);
+    process->closeWriteChannel();
+}
+
 FileType UicGeneratorFactory::sourceType() const
 {
     return FileType::Form;

src/plugins/qtsupport/uicgenerator.h

@@ -43,6 +43,7 @@ protected:
     Utils::FilePath command() const override;
     QStringList arguments() const override;
     ProjectExplorer::FileNameToContentsHash handleProcessFinished(QProcess *process) override;
+    void handleProcessStarted(QProcess *process, const QByteArray &sourceContents) override;
 };
 
 class UicGeneratorFactory : public ProjectExplorer::ExtraCompilerFactory