[ADT] Fix itostr handling of min int64_t value

Summary:
UBSan buildbot caught an undefined behavior in itostr with INT64_MIN.
The negation cannot be represented in the promoted operand (long long).
Negation is well defined on unsigned value though so this commit does
the negation after the static cast.

Reviewers: jhenderson, chandlerc, lattner

Reviewed By: lattner

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82200