[Support] Disable assertion dialogs from the MSVC debug CRT

Summary:
Sets a report hook that emulates pressing "retry" in the "abort, retry,
ignore" dialog box that _CrtDbgReport normally raises.  There are many
other ways to disable assertion reports, but this was the only way I
could find that still calls our exception handler.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D625