[msan] Fix handling of byval arguments with large alignment.

MSan param-tls slots are 8-byte aligned. This change clips
alignment of memcpy into param-tls to 8.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220101 91177308-0d34-0410-b5e6-96231b3b80d8