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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@114192 91177308-0d34-0410-b5e6-96231b3b80d8