Add a -ftrapv-handler= option which allows a handler to invoke instead of...

Add a -ftrapv-handler= option which allows a handler to invoke instead of simply aborting when a signed operation overflows.  This mirrors the (GCC-incompatible) behaviour from clang 1.0 and 1.1 when -ftrapv was specified, but allows the handler to be defined for each compilation unit.

llvm-svn: 114192