Attempt to make buildbot happier with r266032.

Apparently std::numeric_limits<unsigned>::max() isn't constexpr
everywhere yet.

llvm-svn: 266034
diff --git a/llvm/lib/IR/Attributes.cpp b/llvm/lib/IR/Attributes.cpp
index d098fc1..0b77533 100644
--- a/llvm/lib/IR/Attributes.cpp
+++ b/llvm/lib/IR/Attributes.cpp
@@ -38,8 +38,7 @@
 //
 // In order to do this, we need to reserve one value of the second (optional)
 // allocsize argument to signify "not present."
-LLVM_CONSTEXPR static unsigned AllocSizeNumElemsNotPresent =
-    std::numeric_limits<unsigned>::max();
+LLVM_CONSTEXPR static unsigned AllocSizeNumElemsNotPresent = -1;
 
 static uint64_t packAllocSizeArgs(unsigned ElemSizeArg,
                                   const Optional<unsigned> &NumElemsArg) {