Fix XRayTest link on FreeBSD (and likely NetBSD too)

[dim]

Summary

As reported on llvm-testers, during 8.0.0-rc1 testing I got errors while
building of XRayTest, during check-all:

[100%] Generating XRayTest-x86_64-Test
/home/dim/llvm/8.0.0/rc1/Phase3/Release/llvmCore-8.0.0-rc1.obj/./lib/libLLVMSupport.a(Signals.cpp.o): In function `llvm::sys::PrintStackTrace(llvm::raw_ostream&)':
Signals.cpp:(.text._ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x24): undefined reference to `backtrace'
Signals.cpp:(.text._ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x254): undefined reference to `llvm::itaniumDemangle(char const*, char*, unsigned long*, int*)'
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[3]: *** [projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/build.make:73: projects/compiler-rt/lib/xray/tests/unit/XRayTest-x86_64-Test] Error 1
gmake[3]: Target 'projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/build' not remade because of errors.
gmake[2]: *** [CMakeFiles/Makefile2:33513: projects/compiler-rt/lib/xray/tests/unit/CMakeFiles/TXRayTest-x86_64-Test.dir/all] Error 2
gmake[2]: Target 'CMakeFiles/check-all.dir/all' not remade because of errors.
gmake[1]: *** [CMakeFiles/Makefile2:737: CMakeFiles/check-all.dir/rule] Error 2
gmake[1]: Target 'check-all' not remade because of errors.
gmake: *** [Makefile:277: check-all] Error 2
[Release Phase3] check-all failed

This is because the backtrace function requires -lexecinfo on BSD
platforms. To fix this, detect the execinfo library in
cmake/config-ix.cmake, and add it to the unit test link flags.

Additionally, since the code in sys::PrintStackTrace makes use of
itaniumDemangle, also add -lLLVMDemangle. (Note that this is more
of a general problem with libLLVMSupport, but I'm looking for a quick
fix now so it can be merged to the 8.0 branch.)

[krytarowski]

NetBSD is affected in the same way.

[dberris]

LGTM

[mgorny]

Thank you!

compiler-rt/trunk/cmake/config-ix.cmake

 # Symbols.
 check_symbol_exists(__func__ "" COMPILER_RT_HAS_FUNC_SYMBOL)
 
 # Libraries.
 check_library_exists(dl dlopen "" COMPILER_RT_HAS_LIBDL)
 check_library_exists(rt shm_open "" COMPILER_RT_HAS_LIBRT)
 check_library_exists(m pow "" COMPILER_RT_HAS_LIBM)
 check_library_exists(pthread pthread_create "" COMPILER_RT_HAS_LIBPTHREAD)
+check_library_exists(execinfo backtrace "" COMPILER_RT_HAS_LIBEXECINFO)
 
 # Look for terminfo library, used in unittests that depend on LLVMSupport.
 if(LLVM_ENABLE_TERMINFO)
   foreach(library terminfo tinfo curses ncurses ncursesw)
     string(TOUPPER ${library} library_suffix)
     check_library_exists(
       ${library} setupterm "" COMPILER_RT_HAS_TERMINFO_${library_suffix})
     if(COMPILER_RT_HAS_TERMINFO_${library_suffix})

compiler-rt/trunk/lib/xray/tests/CMakeLists.txt

append_list_if(COMPILER_RT_HAS_LLVMTESTINGSUPPORT
       ${LLVM_TESTINGSUPPORT_LIBLIST} XRAY_UNITTEST_LINK_FLAGS)
   else()
     # We add the library directories one at a time in our CFLAGS.
     foreach (DIR ${LLVM_LIBRARY_DIR})
       list(APPEND XRAY_UNITTEST_LINK_FLAGS -L${DIR})
     endforeach()
 
     # We also add the actual libraries to link as dependencies.
-    list(APPEND XRAY_UNITTEST_LINK_FLAGS -lLLVMXRay -lLLVMSupport -lLLVMTestingSupport)
+    list(APPEND XRAY_UNITTEST_LINK_FLAGS -lLLVMXRay -lLLVMSupport -lLLVMDemangle -lLLVMTestingSupport)
   endif()
 
   append_list_if(COMPILER_RT_HAS_LIBM -lm XRAY_UNITTEST_LINK_FLAGS)
   append_list_if(COMPILER_RT_HAS_LIBRT -lrt XRAY_UNITTEST_LINK_FLAGS)
   append_list_if(COMPILER_RT_HAS_LIBDL -ldl XRAY_UNITTEST_LINK_FLAGS)
   append_list_if(COMPILER_RT_HAS_LIBPTHREAD -pthread XRAY_UNITTEST_LINK_FLAGS)
+  append_list_if(COMPILER_RT_HAS_LIBEXECINFO -lexecinfo XRAY_UNITTEST_LINK_FLAGS)
 endif()
 
 macro(add_xray_unittest testname)
   cmake_parse_arguments(TEST "" "" "SOURCES;HEADERS" ${ARGN})
   if(UNIX AND NOT APPLE)
     set(CMAKE_DL_LIBS_INIT "")
     foreach(arch ${XRAY_TEST_ARCH})
       set(TEST_OBJECTS)