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/LibStdcpp.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
index f25391f..695371f 100644
--- a/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibStdcpp.cpp
@@ -298,7 +298,7 @@
         return false;
 
       // Safe to pass nullptr for exe_scope here.
-      auto size = wchar_compiler_type.GetBitSize(nullptr);
+      llvm::Optional<uint64_t> size = wchar_compiler_type.GetBitSize(nullptr);
       if (!size)
         return false;
       const uint32_t wchar_size = *size;