Clang: Add Process Generator

So far we only compiled the precompiled headers only sequentially. The
process generator is creating, managing a queue of processes to compile
the PCHs parallel.

Change-Id: I8075def4ef9e6b0191bbd51b3631d1c51ec7b361
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/testenvironment.h

@@ -45,6 +45,11 @@ public:
         return QString::fromUtf8(CLANG_COMPILER_PATH);
     }
 
+    uint hardwareConcurrency() const
+    {
+        return 2;
+    }
+
 private:
     QTemporaryDir temporaryDirectory;
 };