[new compiler] Fix invalid memory reads when JS closures outlive QML types

[new compiler] Fix invalid memory reads when JS closures outlive QML types

If QQmlCompiledData gets destroyed while somebody still has refcount on the
QV4::CompiledData::CompilationUnit, then unit's _data_ would be freed already
by ~QQmlCompiledData. Given that compilationUnit->data is pointing to the same
malloc'ed address as QQmlCompiledData::qmlUnit, we can just let the
CompilationUnit always own the data.

Fixes tst_qquickloader and makes it possible to run the qquickcomponent tests.

Change-Id: Ie3f3e5335139236d7c2524a327665bda0a9cc847
Reviewed-by: Lars Knoll <lars.knoll@digia.com>