Revert "Roll Clang 214024:216630 (+216684) and switch to CMake (try 2)"

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index b9e8e175de5400f984231f34d2a21bb6c2317834..d66f54c57a0c84cbe2f44c46f87d35a3d74eb599 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -109,17 +109,6 @@ config("compiler") {
       # and the fix is in chromium.
       "-fno-slp-vectorize",
     ]
-
-    if (is_debug) {
-      # Allow comparing the address of references and 'this' against 0
-      # in debug builds. Technically, these can never be null in
-      # well-defined C/C++ and Clang can optimize such checks away in
-      # release builds, but they may be used in asserts in debug builds.
-      cflags_cc += [
-        "-Wno-undefined-bool-conversion",
-        "-Wno-tautological-undefined-compare",
-      ]
-    }
   }
 
   if (is_clang && !is_win) {