[libc++] Build the dylib with sanitizers when requested (#73656)

We were detecting which sanitizer flags to use when building
libc++.dylib but we were never actually adding those flags to the
targets, which means that our sanitized builds would basically build the
dylib without any sanitizers enabled.

libcxx/CMakeLists.txt

@@ -621,10 +621,10 @@ function(get_sanitizer_flags OUT_VAR  USE_SANITIZER)
         append_flags(SANITIZER_FLAGS "-fsanitize-memory-track-origins")
       endif()
     elseif (USE_SANITIZER STREQUAL "Undefined")
-      append_flags(SANITIZER_FLAGS "-fsanitize=undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all")
+      append_flags(SANITIZER_FLAGS "-fsanitize=undefined" "-fno-sanitize=vptr,function" "-fno-sanitize-recover=all")
     elseif (USE_SANITIZER STREQUAL "Address;Undefined" OR
             USE_SANITIZER STREQUAL "Undefined;Address")
-      append_flags(SANITIZER_FLAGS "-fsanitize=address,undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all")
+      append_flags(SANITIZER_FLAGS "-fsanitize=address,undefined" "-fno-sanitize=vptr,function" "-fno-sanitize-recover=all")
     elseif (USE_SANITIZER STREQUAL "Thread")
       append_flags(SANITIZER_FLAGS -fsanitize=thread)
     elseif (USE_SANITIZER STREQUAL "DataFlow")
@@ -639,6 +639,8 @@ function(get_sanitizer_flags OUT_VAR  USE_SANITIZER)
 endfunction()
 
 get_sanitizer_flags(SANITIZER_FLAGS "${LLVM_USE_SANITIZER}")
+add_library(cxx-sanitizer-flags INTERFACE)
+target_compile_options(cxx-sanitizer-flags INTERFACE ${SANITIZER_FLAGS})
 
 # Link system libraries =======================================================
 function(cxx_link_system_libraries target)
@@ -821,6 +823,7 @@ function(cxx_add_common_build_flags target)
   cxx_add_rtti_flags(${target})
   cxx_add_module_flags(${target})
   cxx_link_system_libraries(${target})
+  target_link_libraries(${target} PRIVATE cxx-sanitizer-flags)
 endfunction()
 
 #===============================================================================