Speed up opening .pro files on windows for some users

If the build directory is empty, which can happen if we are parsing
a completely unconfigured project, then UIDIR might end up being '/'.

The uiHeaderFile function appeneded /ui_XXX.h to that, leading to
a network path.

Fix that by ensuring in various places that we handle this case.

Task-number: QTCREATORBUG-10733
Change-Id: Ib848ed4efbeacc6ee7ff862bef861a58f6d3ea34
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -896,7 +896,9 @@ QString QmakeProject::generatedUiHeader(const FileName &formFile) const
     // the top-level project only.
     if (m_rootProjectNode)
         if (const QmakeProFileNode *pro = proFileNodeOf(m_rootProjectNode, FormType, formFile))
-            return QmakeProFileNode::uiHeaderFile(pro->uiDirectory(pro->buildDir()), formFile);
+            return QmakeProFileNode::uiHeaderFile(
+                        pro->uiDirectory(Utils::FileName::fromString(pro->buildDir())),
+                        formFile);
     return QString();
 }