[cmake] Pass -Wl,-z,nodelete on Linux to prevent unloading

[mgorny]

Summary

Prevent unloading shared libraries on Linux when dlclose() is called.
This is necessary since command-line option parsing API relies on
registering the global option instances in the option parser instance
which can be loaded in a different shared library.

Given that we can't reliably remove those options when a library is
unloaded, the parser ends up containing dangling references. Since glibc
has relatively complex library unloading rules, some of the LLVM
libraries can be unloaded while others (including the Support library)
stay loaded causing quite a mayhem. To reliably prevent that, just
forbid unloading all libraries -- it's a very bad idea anyway.

While the issue arguably happens only with BUILD_SHARED_LIBS, it may
affect any library reusing llvm::cl interface.

Based on patch provided Ross Hayward on https://bugs.gentoo.org/617154.
Previously hit by Fedora back in Feb 2016:
https://lists.freedesktop.org/archives/mesa-dev/2016-February/107242.html

[beanz]

LGTM

[mgorny]

Updated to fix unittest failure.

[beanz]

LGTM.

cmake/modules/HandleLLVMOptions.cmake

 # build might work on ELF but fail on MachO/COFF.
 if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" OR WIN32 OR CYGWIN OR
         ${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR
         ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD") AND
    NOT LLVM_USE_SANITIZER)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
 endif()
 
+# Pass -Wl,-z,nodelete. This makes sure our shared libraries are not unloaded
+# by dlclose(). We need that since the CLI API relies on cross-references
+# between global objects which became horribly broken when one of the libraries
+# is unloaded.
+if(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
+  set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,nodelete")
+endif()
+
 
 function(append value)
   foreach(variable ${ARGN})
     set(${variable} "${${variable}} ${value}" PARENT_SCOPE)
   endforeach(variable)
 endfunction()
 
 function(append_if condition value)