Fix alignof computation of large arrays on x86_64.

We were exposing the extra alignment given to large arrays. The new behavior
matches gcc, which is a good thing since this is a gcc extension.

Thanks to Joerg Sonnenberger for noticing it.

While at it, centralize the method description in the .h file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187999 91177308-0d34-0410-b5e6-96231b3b80d8