[CMake] Don't optimize tests so much under ThinLTO

[aeubanks]

Summary

This drops check-llvm under -DLLVM_ENABLE_LTO=Thin from 13m to 10m20s on Windows and 20m to 15m35s on Linux.

llvm/cmake/modules/AddLLVM.cmake

function(add_unittest test_suite test_name)
   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")
+    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()
+
   set(outdir ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR})
   set_output_directory(${test_name} BINARY_DIR ${outdir} LIBRARY_DIR ${outdir})
   # libpthreads overrides some standard library symbols, so main
   # executable must be linked with it in order to provide consistent
   # API for all shared libaries loaded by this executable.
   target_link_libraries(${test_name} PRIVATE gtest_main gtest ${LLVM_PTHREAD_LIB})
 
   add_dependencies(${test_suite} ${test_name})