Enable checked builds of CoreCLR.

Enable checked builds of CoreCLR.

In checked builds coreclr, mscorlib, and the test are built optimized
but assertion checking is on. This adds additional coverage (the jit is
optimizing and assertion checking is on), speeds up testing compared to debug,
and allows testing JIT stress modes.

This doesn't affect CoreFX.

Several tests are currently failing in checked configuration due to newly
discovered  bugs (JIT asserts). We didn't see these asserts in debug mode
because by default JIT is in minopt mode; we didn't see these bugs in release
mode because assertion checking is off. I will file the bugs once checked build
changes are in.