Replace LLVM_ALIGNAS with alignas as a follow-up of r337330

The minimum required GCC version was raised to 4.8 (which started to support alignas) in r284497.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338099 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed