[llvm] [CodeGen] Expose the extensibility of PassConfig to plugins (PR #139059)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

Failed to link the unittest:
```
[715/716] Linking CXX executable unittests/CodeGen/CGPluginTest/CGPluginTest
FAILED: unittests/CodeGen/CGPluginTest/CGPluginTest 
: && /usr/bin/c++ -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-uninitialized -Wno-nonnull -Wno-class-memaccess -Wno-redundant-move -Wno-pessimizing-move -Wno-noexcept-type -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wno-misleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -Wl,--export-dynamic   -Wl,--gc-sections unittests/CodeGen/CGPluginTest/CMakeFiles/CGPluginTest.dir/PluginTest.cpp.o unittests/CodeGen/CGPluginTest/CMakeFiles/CGPluginTest.dir/Plugin/CodeGenTestPass.cpp.o -o unittests/CodeGen/CGPluginTest/CGPluginTest  -Wl,-rpath,/home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/lib  lib/libLLVMX86CodeGen.so.21.0git  lib/libLLVMX86AsmParser.so.21.0git  lib/libLLVMX86Desc.so.21.0git  lib/libLLVMX86Disassembler.so.21.0git  lib/libLLVMX86Info.so.21.0git  lib/libLLVMAMDGPUCodeGen.so.21.0git  lib/libLLVMAMDGPUAsmParser.so.21.0git  lib/libLLVMAMDGPUDisassembler.so.21.0git  lib/libllvm_gtest_main.so.21.0git  lib/libLLVMTestingSupport.so.21.0git  lib/libLLVMCodeGen.so.21.0git  lib/libLLVMTarget.so.21.0git  lib/libLLVMAMDGPUDesc.so.21.0git  lib/libLLVMAMDGPUInfo.so.21.0git  lib/libLLVMAMDGPUUtils.so.21.0git  lib/libLLVMCore.so.21.0git  lib/libllvm_gtest.so.21.0git  lib/libLLVMSupport.so.21.0git  -Wl,-rpath-link,/home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/lib && :
/usr/bin/ld: unittests/CodeGen/CGPluginTest/CMakeFiles/CGPluginTest.dir/PluginTest.cpp.o: undefined reference to symbol '_ZN4llvm9MCContextC1ERKNS_6TripleEPKNS_9MCAsmInfoEPKNS_14MCRegisterInfoEPKNS_15MCSubtargetInfoEPKNS_9SourceMgrEPKNS_15MCTargetOptionsEbNS_9StringRefE'
/usr/bin/ld: /home/botworker/builds/openmp-offload-amdgpu-runtime-2/llvm.build/lib/libLLVMMC.so.21.0git: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```

That's interesting, because there are other unittests that do pretty much the same. Maybe they don't build in this config.

https://github.com/llvm/llvm-project/pull/139059