Reenable C4800 and C4805 compiler warnings as most of these have
been fixed. Disable C4267 and C4244 as these roughly correspond
to GCC's -Wconversion which we have not enabled on other platforms.

Disable a few warnings for compilation of code in /extra.

Change-Id: I90f02f5fb686734cae5657d0d46dc18ca1171376
(cherry picked from commit bb9f32f3287aef600038e66da2a540915da1fb37)