Replace auto -> llvm::Optional<uint64_t>

This addresses post-commit feedback for https://reviews.llvm.org/D56688

llvm-svn: 351237
diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxxBitset.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibCxxBitset.cpp
index 0d39e5a..489ac4d 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibCxxBitset.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxxBitset.cpp
@@ -80,7 +80,8 @@
   ValueObjectSP chunk;
   // For small bitsets __first_ is not an array, but a plain size_t.
   if (m_first->GetCompilerType().IsArrayType(&type, nullptr, nullptr)) {
-    auto bit_size = type.GetBitSize(ctx.GetBestExecutionContextScope());
+    llvm::Optional<uint64_t> bit_size =
+        type.GetBitSize(ctx.GetBestExecutionContextScope());
     if (!bit_size || *bit_size == 0)
       return {};
     chunk = m_first->GetChildAtIndex(idx / *bit_size, true);
@@ -91,7 +92,8 @@
   if (!type || !chunk)
     return {};
 
-  auto bit_size = type.GetBitSize(ctx.GetBestExecutionContextScope());
+  llvm::Optional<uint64_t> bit_size =
+      type.GetBitSize(ctx.GetBestExecutionContextScope());
   if (!bit_size || *bit_size == 0)
     return {};
   size_t chunk_idx = idx % *bit_size;