We need to make sure to set the right destination for the ExamplesIRTransforms
library target and respect LLVM_BUILD_EXAMPLES. This patch adds a
add_llvm_example_library macro similar to the add_llvm_example one,
which should set the destination for the install target properly and
exclude it from the ALL target if LLVM_BUILD_EXAMPLES=Off.
It should fix a problem where using LLVMExports.cmake fails, because
it expects libExampleIRTransforms.a to be installed.