[lldb/DataFormatter] Check for overflow when finding NSDate epoch

Summary:
Fixes UBSan-reported issues where the date value inside of an
uninitialized NSDate overflows the 64-bit epoch.

rdar://61774575

Reviewers: JDevlieghere, mib, teemperor

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D80150