[libc] fix default ASAN behavior when SCUDO is enabled

[michaelrj]

Summary

if COMPILER_RT_BUILD_GWP_ASAN isn't set, then in compiler-rt it defaults
to on, but since it isn't defined in llvm libc, it was previously
defaulting to off, causing build errors from the mismatch. This fixes
the default behavior by making it default to on in llvm libc.

libc/CMakeLists.txt

message(WARNING "
     (pass -DLLVM_LIBC_ENABLE_LINTING=ON to cmake).")
 endif()
 
 option(LLVM_LIBC_INCLUDE_SCUDO "Include the SCUDO standalone as the allocator for LLVM libc" OFF)
 if(LLVM_LIBC_INCLUDE_SCUDO)
   if (NOT "compiler-rt" IN_LIST LLVM_ENABLE_PROJECTS)
     message(FATAL_ERROR "SCUDO cannot be included without adding compiler-rt to LLVM_ENABLE_PROJECTS")
   endif()
+  if(NOT DEFINED COMPILER_RT_BUILD_GWP_ASAN)
+    set(COMPILER_RT_BUILD_GWP_ASAN ON)
+  endif()
 endif()
 
 include(CMakeParseArguments)
 include(LLVMLibCRules)
 include(LLVMLibCCheckCpuFeatures)
 
 if(EXISTS "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/entrypoints.txt")
   set(entrypoint_file "${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/entrypoints.txt")