[ORC] Use ExecutionSession's pre-constructed main JITDylib in LLJIT.

As of r342086 ExecutionSession automatically creates a 'main' JITDylib, so
there is no need for LLJIT to create its own.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343167 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed