Summary:
Create a separate page describing UBSan tool, move the description of
fine-grained checks there, provide extra information about supported
platforms, symbolization etc. This text is compiled from four parts:

* Existing documentation copied from User's Manual
* Layout used in documentation for another sanitizers (ASan, MSan etc.)
* Text written from scratch
* Small parts taken from Michael Morrison's attempt at creating UBSan
  page:
  http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20141215/249503.html

Reviewers: kcc, rsmith, silvas

Subscribers: tberghammer, danalbert, srhines, kcc

Differential Revision: http://reviews.llvm.org/D15217

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254733 91177308-0d34-0410-b5e6-96231b3b80d8