Revert "[ORC] Make MaterializationResponsibility immovable, pass by unique_ptr."

This reverts commit c74900ca.

This appears to be breaking some builds on macOS and has been causing
build failures on Green Dragon (see below). I am reverting this for now,
to unblock testing on Green Dragon.

http://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental/18144/console

[65/187] /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++  -DBUILD_EXAMPLES -DGTEST_HAS_RTTI=0 -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Iexamples/ThinLtoJIT -I/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT -Iinclude -I/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O3  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.9    -fno-exceptions -fno-rtti -UNDEBUG -std=c++14 -MD -MT examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o -MF examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o.d -o examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o -c /Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT/ThinLtoDiscoveryThread.cpp
FAILED: examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++  -DBUILD_EXAMPLES -DGTEST_HAS_RTTI=0 -D_DEBUG -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -Iexamples/ThinLtoJIT -I/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT -Iinclude -I/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/include -fPIC -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wstring-conversion -fdiagnostics-color -O3  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -mmacosx-version-min=10.9    -fno-exceptions -fno-rtti -UNDEBUG -std=c++14 -MD -MT examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o -MF examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o.d -o examples/ThinLtoJIT/CMakeFiles/ThinLtoJIT.dir/ThinLtoDiscoveryThread.cpp.o -c /Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT/ThinLtoDiscoveryThread.cpp
In file included from /Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT/ThinLtoDiscoveryThread.cpp:7:
/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/examples/ThinLtoJIT/ThinLtoInstrumentationLayer.h:37:68: error: non-virtual member function marked 'override' hides virtual member function
  void emit(MaterializationResponsibility R, ThreadSafeModule TSM) override;
                                                                   ^
/Users/buildslave/jenkins/workspace/clang-stage1-cmake-RA-incremental/llvm-project/llvm/include/llvm/ExecutionEngine/Orc/Layer.h:103:16: note: hidden overloaded virtual function 'llvm::orc::IRLayer::emit' declared here: type mismatch at 1st parameter ('std::unique_ptr<MaterializationResponsibility>' vs 'llvm::orc::MaterializationResponsibility')
  virtual void emit(std::unique_ptr<MaterializationResponsibility> R,
               ^
1 error generated.