[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

llvm/include/llvm/ADT/StringExtras.h

@@ -245,7 +245,7 @@ inline std::string utostr(uint64_t X, bool isNeg = false) {
 
 inline std::string itostr(int64_t X) {
   if (X < 0)
-    return utostr(static_cast<uint64_t>(-X), true);
+    return utostr(-static_cast<uint64_t>(X), true);
   else
     return utostr(static_cast<uint64_t>(X));
 }