Separate out the initialization kind for a statement expression result

Separate out the initialization kind for a statement expression result
from that for a return value.

No functionality change intended: I don't believe any of the diagnostics
affected by this patch are reachable when initializing the result of
statement expression.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@337728 91177308-0d34-0410-b5e6-96231b3b80d8