constexpr: remove integral conversion overflow checking introduced in r149286.
As Eli points out, this is implementation-defined, and the way we define it
makes this fine.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149327 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed