Add a method to disable the Windows crash / assert dialogs.

When running the test suite on Windows, we can't have Windows popping
up dialogs when LLDB crashes in native code because it will hang
the test suite.  This patch silences those dialogs by checking an
environment variable at startup and configuring Windows based on
its value.

This patch also adds an environment variable to force inferiors to
never spawn in their own console window.  This is useful to prevent
new window spawm when running the test suite.

Reviewed by: Scott Graham

Differential Revision: http://reviews.llvm.org/D6628