patch: make ubsan report error summaries

[Nick Lewycky]

The attached patch makes ubsan emit summaries of errors it encounters. The
format of these summaries is:
  UndefinedBehaviourSanitizer: signed-integer-overflow file:49:7
where the string is the flag name. Most of the patch is adding the flag
names to all the reports all over.

This patch is stacked on top of
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20131021/091535.html
,
or else ubsan's tests will fail.

Please review!

Nick
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