[CMake] Use -O0 for unittests under full LTO as well

[phosek]

Summary

We already use -O0 for unittests under ThinLTO, do the same for full LTO
where the compile time costs to runtime benefits tradeoff is even worse.

[smeenai]

LGTM, makes a lot of sense.

llvm/cmake/modules/AddLLVM.cmake

if(SUPPORTS_GNU_ZERO_VARIADIC_MACRO_ARGUMENTS_FLAG)
     list(APPEND LLVM_COMPILE_FLAGS "-Wno-gnu-zero-variadic-macro-arguments")
   endif()
 
   set(LLVM_REQUIRES_RTTI OFF)
 
   list(APPEND LLVM_LINK_COMPONENTS Support) # gtest needs it for raw_ostream
   add_llvm_executable(${test_name} IGNORE_EXTERNALIZE_DEBUGINFO NO_INSTALL_RPATH ${ARGN})
 
-  # The runtime benefits of ThinLTO don't outweight the compile time costs for tests.
-  if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
+  # The runtime benefits of LTO don't outweight the compile time costs for tests.
+  if(LLVM_ENABLE_LTO)
     if((UNIX OR MINGW) AND LLVM_USE_LINKER STREQUAL "lld")
       set_property(TARGET ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " -Wl,--lto-O0")
     elseif(LINKER_IS_LLD_LINK)
       set_property(TARGET ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " /opt:lldlto=0")
     endif()
   endif()