[CMake] Let llvm-shlib work on Linux with --whole-archive.
FIXME: It should work on not only Linux but elf-targeting gnu ld.
For example if LLVM_DYLIB_COMPONENTS is "BitWriter Support", CMake emits the command line like;
-Wl,--whole-archive
lib/libLLVMBitWriter.a
lib/libLLVMSupport.a *1
-Wl,--no-whole-archive
lib/libLLVMCore.a
lib/libLLVMSupport.a *2
-lrt -ldl -ltinfo -lpthread -lm
It works since symbols in LLVMCore is resolved with not *2 but *1.
Unfortunately, --gc-sections is not powerful in this case to prune unused "visibility(default)" entries.
I am still experimenting other way not to rely on --whole-archive.
llvm-svn: 221591