[ExecutionContext] Return the target/process byte order.

Currently ExecutionContext::GetByteOrder() always returns the host byte
order. This seems like a simple mistake: the return keyword appears to
have been omitted by accident. This patch fixes that and adds a unit
test.

Bugreport: https://llvm.org/PR37950

Differential revision: https://reviews.llvm.org/D48704

llvm-svn: 368181