[LLVM] Plugins broken on AIX

[aengelke]

Tests that use pass plugins on AIX are broken and fail, e.g., with:

# .---command stderr------------
# | LLVM ERROR: Could not load library '/home/llvm/llvm-external-buildbots/workers/aix-ppc64/clang-ppc64-aix/build/lib/ExampleIRTransforms.so': rtld: 0712-001 Symbol _ZN4llvm21GenericDomTreeUpdaterINS_14DomTreeUpdaterENS_13DominatorTreeENS_17PostDominatorTreeEE16applyUpdatesImplILb1EEEvv was referenced
# |       from module /home/llvm/llvm-external-buildbots/workers/aix-ppc64/clang-ppc64-aix/build/lib/ExampleIRTransforms.so(), but a runtime definition
# | 	    of the symbol was not found.
# | rtld: 0712-001 Symbol _ZN4llvm21GenericDomTreeUpdaterINS_14DomTreeUpdaterENS_13DominatorTreeENS_17PostDominatorTreeEE16applyUpdatesImplILb0EEEvv was referenced
# |       from module /home/llvm/llvm-external-buildbots/workers/aix-ppc64/clang-ppc64-aix/build/lib/ExampleIRTransforms.so(), but a runtime definition
# | 	    of the symbol was not found.
# | 
# `-----------------------------

I disabled the relevant tests (Feature/load_extension.ll, Examples/IRTransforms) in 249acb6 and 66d92d4.