[ORC] Add a 'lookup' convenience function for finding symbols in a list of VSOs.

The lookup function takes a list of VSOs, a set of symbol names (or just one
symbol name) and a materialization function object. It returns an
Expected<SymbolMap> (if given a set of names) or an Expected<JITEvaluatedSymbol>
(if given just one name). The lookup method constructs an
AsynchronousSymbolQuery for the given names, applies that query to each VSO in
the list in turn, and then blocks waiting for the query to complete. If
threading is enabled then the materialization function object can be used to
execute the materialization on different threads. If threading is disabled the
MaterializeOnCurrentThread utility must be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327474 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed