[libc++][hardening][NFC] Remove the `LIBCXX_DEBUG_BUILD` CMake variable.

[var-const]

Summary

It's no longer necessary to define _DEBUG because we use our own
internal assertion mechanism in libc++.

[ldionne]

LGTM w/ comment.

libcxx/CMakeLists.txt

482–483

Can you move this closer to the first use of the variable?

[var-const]

Address the comment

libcxx/CMakeLists.txt

 # Configure compiler.
 include(config-ix)
 
 # Configure coverage options.
 if (LIBCXX_GENERATE_COVERAGE)
   include(CodeCoverage)
   set(CMAKE_BUILD_TYPE "COVERAGE" CACHE STRING "" FORCE)
 endif()
 
-string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
-if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
-  set(LIBCXX_DEBUG_BUILD ON)
-else()
-  set(LIBCXX_DEBUG_BUILD OFF)
-endif()
-
 #===============================================================================
 # Setup Compiler Flags
 #===============================================================================
 
 include(HandleLibCXXABI) # Setup the ABI library flags
 
 # FIXME: Remove all debug flags and flags that change which Windows
 # default libraries are linked. Currently we only support linking the
 # non-debug DLLs
if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY AND LIBCXX_ENABLE_SHARED)
   if (APPLE)
     add_link_flags("-undefined dynamic_lookup")
   else()
     # Relax this restriction from HandleLLVMOptions
     string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
   endif()
 endif()
 
-# Assertion flags =============================================================
-define_if(LIBCXX_DEBUG_BUILD -D_DEBUG)
-if (LIBCXX_ENABLE_ASSERTIONS AND NOT LIBCXX_DEBUG_BUILD)
-  # MSVC doesn't like _DEBUG on release builds. See PR 4379.
-  define_if_not(MSVC -D_DEBUG)
-endif()
-
 # Modules flags ===============================================================
 # FIXME The libc++ sources are fundamentally non-modular. They need special
 # versions of the headers in order to provide C++03 and legacy ABI definitions.
 # NOTE: The public headers can be used with modules in all other contexts.
 function(cxx_add_module_flags target)
   if (LLVM_ENABLE_MODULES)
     # Ignore that the rest of the modules flags are now unused.
     target_add_compile_flags_if_supported(${target} PUBLIC -Wno-unused-command-line-argument)
     target_compile_options(${target} PUBLIC -fno-modules)
   endif()
 endfunction()
 
+string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
+
 # Sanitizer flags =============================================================
 
 function(get_sanitizer_flags OUT_VAR  USE_SANITIZER)
   set(SANITIZER_FLAGS)
   set(USE_SANITIZER "${USE_SANITIZER}")
   # NOTE: LLVM_USE_SANITIZER checks for a UNIX like system instead of MSVC.
   # But we don't have LLVM_ON_UNIX so checking for MSVC is the best we can do.
   if (USE_SANITIZER AND NOT MSVC)
if (LIBCXX_HAS_ATOMIC_LIB)
     target_link_libraries(${target} PRIVATE atomic)
   endif()
 
   if (MINGW)
     target_link_libraries(${target} PRIVATE "${MINGW_LIBRARIES}")
   endif()
 
   if (MSVC)
-    if (LIBCXX_DEBUG_BUILD)
+    if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
       set(LIB_SUFFIX "d")
     else()
       set(LIB_SUFFIX "")
     endif()
 
     target_link_libraries(${target} PRIVATE ucrt${LIB_SUFFIX}) # Universal C runtime
     target_link_libraries(${target} PRIVATE vcruntime${LIB_SUFFIX}) # C++ runtime
     target_link_libraries(${target} PRIVATE msvcrt${LIB_SUFFIX}) # C runtime startup files