[libcxx] Fix PR21580 - Undefined behavior in readEncodedPointer()

Summary: This patch fixes a bug in `readEncodedPointer()` where it would read from memory that was not suitably aligned. This patch fixes it by using memcpy.

Reviewers: danalbert, echristo, compnerd, mclow.lists

Reviewed By: compnerd, mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D8179

llvm-svn: 231839