[DataLayout] Check StackNatural and FunctionPtr alignments.

MaybeAlignment asserts that the passed in value is == 0 or a power of 2.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=16272

Reviewers: michaelplatings, gchatelet, jakehehrlich, jfb

Reviewed By: gchatelet

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65858

llvm-svn: 368191