IRgen: Add CreateMemTemp, for creating an temporary memory object for a particular type, and flood fill. - CreateMemTemp sets the alignment on the alloca correctly, which fixes a great many places in IRgen where we were doing the wrong thing.

- This fixes many many more places than the test case, but my feeling is we need to audit alignment systematically so I'm not inclined to try hard to test the individual fixes in this patch. If this bothers you, patches welcome!

PR6240.

llvm-svn: 95648