[libc++] Fix typos causing compilation errors when _LIBCPP_DEBUG_LEVEL >= 2

Summary: This patch fixes a couple of typos that cause compilation errors when application includes <unordered_map> and enables the libc++'s debugging capabilities.

Reviewers: EricWF

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24883

llvm-svn: 282446