[ubsan] Add a check for pointer overflow UB

Check pointer arithmetic for overflow.

For some more background on this check, see:

  https://wdtz.org/catching-pointer-overflow-bugs.html
  https://reviews.llvm.org/D20322

Patch by Will Dietz and John Regehr!

This version of the patch is different from the original in a few ways:

  - It introduces the EmitCheckedInBoundsGEP utility which inserts
    checks when the pointer overflow check is enabled.

  - It does some constant-folding to reduce instrumentation overhead.

  - It does not check some GEPs in CGExprCXX. I'm not sure that
    inserting checks here, or in CGClass, would catch many bugs.

Possible future directions for this check:

  - Introduce CGF.EmitCheckedStructGEP, to detect overflows when
    accessing structures.

Testing: Apart from the added lit test, I ran check-llvm and check-clang
with a stage2, ubsan-instrumented clang. Will and John have also done
extensive testing on numerous open source projects.

Differential Revision: https://reviews.llvm.org/D33305

llvm-svn: 304459