[ubsan] array-bounds: Ignore params with constant size

[ubsan] array-bounds: Ignore params with constant size

This is a follow-up to r320128. Eli pointed out that there is some gray
area in the language standard about whether the constant size is exact,
or a lower bound.

https://reviews.llvm.org/D40940

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320185 91177308-0d34-0410-b5e6-96231b3b80d8