Enable -Wsuggest-override in the LLVM build

This patch adds Clang's new (and GCC's old) -Wsuggest-override to the warning flags for the LLVM build. The warning is a stronger form of -Winconsistent-missing-override which warns _everywhere_ that override is missing, not just in places where it's inconsistent within a class.

Some directories in the monorepo need the warning disabled for compatibility's, or sanity's, sake; in particular, libcxx/libcxxabi, and any code implementing or interoperating with googletest, googlemock, or google benchmark (which do not themselves use override). This patch adds -Wno-suggest-override to the relevant CMakeLists.txt's to accomplish this.

Differential Revision: https://reviews.llvm.org/D84126

cmake/modules/HandleLLVMOptions.cmake

@@ -672,6 +672,9 @@ if (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
   # Enable -Wdelete-non-virtual-dtor if available.
   add_flag_if_supported("-Wdelete-non-virtual-dtor" DELETE_NON_VIRTUAL_DTOR_FLAG)
 
+  # Enable -Wsuggest-override if available.
+  add_flag_if_supported("-Wsuggest-override" SUGGEST_OVERRIDE_FLAG)
+
   # Check if -Wcomment is OK with an // comment ending with '\' if the next
   # line is also a // comment.
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})

utils/benchmark/CMakeLists.txt

@@ -156,6 +156,10 @@ else()
     add_cxx_compiler_flag(-fno-exceptions)
   endif()
 
+  if (CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG)
+    add_cxx_compiler_flag(-Wno-suggest-override)
+  endif()
+
   if (HAVE_CXX_FLAG_FSTRICT_ALIASING)
     if (NOT CMAKE_CXX_COMPILER_ID STREQUAL "Intel") #ICC17u2: Many false positives for Wstrict-aliasing
       add_cxx_compiler_flag(-Wstrict-aliasing)

utils/unittest/CMakeLists.txt

@@ -43,6 +43,9 @@ endif()
 if(CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG)
   add_definitions("-Wno-covered-switch-default")
 endif()
+if(CXX_SUPPORTS_SUGGEST_OVERRIDE_FLAG)
+  add_definitions("-Wno-suggest-override")
+endif()
 
 set(LLVM_REQUIRES_RTTI 1)
 add_definitions( -DGTEST_HAS_RTTI=0 )