Merge two attribute diagnostics into one

Summary:
Merged the recently added `err_attribute_argument_negative` diagnostic
with existing `err_attribute_requires_positive_integer` diagnostic:
the former allows only strictly positive integer, while the latter
also allows zero.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D51853

llvm-svn: 342367
diff --git a/clang/lib/Sema/SemaStmtAttr.cpp b/clang/lib/Sema/SemaStmtAttr.cpp
index ba96ba9..c720aff 100644
--- a/clang/lib/Sema/SemaStmtAttr.cpp
+++ b/clang/lib/Sema/SemaStmtAttr.cpp
@@ -304,7 +304,7 @@
     if (Val <= 0) {
       S.Diag(A.getRange().getBegin(),
              diag::err_attribute_requires_positive_integer)
-          << A;
+          << A << /* positive */ 0;
       return nullptr;
     }
     UnrollFactor = Val;