[ORC] Fix a data race in the lookup function.

[ORC] Fix a data race in the lookup function.

The Error locals need to be protected by a mutex. (This could be fixed by
having the promises / futures contain Expected and Error values, but
MSVC's future implementation does not support this yet).

Hopefully this will fix some of the errors seen on the builders due to
r327474.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327477 91177308-0d34-0410-b5e6-96231b3b80d8