[CodeGen] Provide an appropriate alignment for dynamic allocas

[CodeGen] Provide an appropriate alignment for dynamic allocas

GCC documents __builtin_alloca as aligning the storage to at least
__BIGGEST_ALIGNMENT__.

MSVC documents essentially the same for the x64 ABI:
https://msdn.microsoft.com/en-us/library/x9sx5da1.aspx

The 32-bit ABI follows the same rule: it emits a call to _alloca_probe_16

Differential Revision: https://reviews.llvm.org/D24378

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285316 91177308-0d34-0410-b5e6-96231b3b80d8