Added alignof intrinsic support: new tests and docs.

Also added test case from issue #1946.

[SVN r46128]

libs/type_traits

@@ -1 +1 @@
-Subproject commit 3cc8416611ea8e784d8bd56773a051a5eac483bc
+Subproject commit 21d466f4513744bbe058e79d3825dca4e3fb947e