[ORC] Use ExecutionSession's pre-constructed main JITDylib in LLJIT.
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@343167 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm