[clang][NFC] Fix typo in the name of a note

Summary:
r306722 introduced a new note called note_silence_unligned_allocation_unavailable
where I believe what was meant is note_silence_aligned_allocation_unavailable.

Reviewers: ahatanak

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51043

llvm-svn: 340288