[msan] Fix handling of array types.

Switch array type shadow from a single integer to
an array of integers (i.e. make it per-element).
This simplifies instrumentation of extractvalue and fixes PR20493.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214398 91177308-0d34-0410-b5e6-96231b3b80d8