Disable GUI dialogs on Windows by default

Disable GUI dialogs on Windows by default

This change disables creating pop-up dialog boxes by default on Windows
when assertions fail.  The change consists of two pieces:

1) Make COMPlus_NoGuiOnAssert effective in Release builds (currently only
Debug/Checked builds)
2) Make DebuggerAssert::ShowDefaultAssertDialog honor the
COMPlus_NoGuiOnAssert variable.
3) Make COMPlus_NoGuiOnAssert=1 by default.

Note that COMPlus_NoGuiOnAssert was already honored for assertions
originating from native code such as the JIT.  If pop-ups are desired,
one can still set COMPlus_NoGuiOnAssert=0.

Fixes #7678