Speed up construction of basic QVariants

Speed up construction of basic QVariants

Instead of first creating an initialized Private struct, then overwrite
the member variables, we added an internal constructor that
initializes the private struct sanely.

In the new (inlined, internal) constructor, both the MSB bits in the
bitfield are 0, and since the value of internal meta-type ID is so
low that the two MSB bits should never be set, the compiler can
(hopefully) optimize away the bit-fiddling initialization of the bit-field.

Callgrind shows about 33% speed-up in e.g. QVariant::QVariant(int)

Change-Id: I706773a71c0d8dcbe119ad15411578b81892deb5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>