Revert "[-Wunterminated-string-initialization] Handle C string literals ending with explicit '\0' (#143487)"

This reverts commit 9903c19.

Caused the following buildbot failure:
https://lab.llvm.org/buildbot/#/builders/144/builds/28591

Please fix before recommitting.

Author: Weaver

clang/docs/ReleaseNotes.rst

@@ -222,7 +222,6 @@ C Language Changes
 
     char buf1[3] = "foo"; // -Wunterminated-string-initialization
     char buf2[3] = "flarp"; // -Wexcess-initializers
-    char buf3[3] = "fo\0";  // This is fine, no warning.
 
   This diagnostic can be suppressed by adding the new ``nonstring`` attribute
   to the field or variable being initialized. #GH137705

clang/lib/Sema/SemaInit.cpp

@@ -260,37 +260,29 @@ static void CheckStringInit(Expr *Str, QualType &DeclT, const ArrayType *AT,
              diag::ext_initializer_string_for_char_array_too_long)
           << Str->getSourceRange();
     else if (StrLength - 1 == ArrayLen) {
-      // In C, if the string literal is null-terminated explicitly, e.g., `char
-      // a[4] = "ABC\0"`, there should be no warning:
-      const auto *SL = dyn_cast<StringLiteral>(Str->IgnoreParens());
-      bool IsSLSafe = SL && SL->getLength() > 0 &&
-                      SL->getCodeUnit(SL->getLength() - 1) == 0;
-
-      if (!IsSLSafe) {
-        // If the entity being initialized has the nonstring attribute, then
-        // silence the "missing nonstring" diagnostic. If there's no entity,
-        // check whether we're initializing an array of arrays; if so, walk the
-        // parents to find an entity.
-        auto FindCorrectEntity =
-            [](const InitializedEntity *Entity) -> const ValueDecl * {
-          while (Entity) {
-            if (const ValueDecl *VD = Entity->getDecl())
-              return VD;
-            if (!Entity->getType()->isArrayType())
-              return nullptr;
-            Entity = Entity->getParent();
-          }
+      // If the entity being initialized has the nonstring attribute, then
+      // silence the "missing nonstring" diagnostic. If there's no entity,
+      // check whether we're initializing an array of arrays; if so, walk the
+      // parents to find an entity.
+      auto FindCorrectEntity =
+          [](const InitializedEntity *Entity) -> const ValueDecl * {
+        while (Entity) {
+          if (const ValueDecl *VD = Entity->getDecl())
+            return VD;
+          if (!Entity->getType()->isArrayType())
+            return nullptr;
+          Entity = Entity->getParent();
+        }
+
+        return nullptr;
+      };
+      if (const ValueDecl *D = FindCorrectEntity(&Entity);
+          !D || !D->hasAttr<NonStringAttr>())
+        S.Diag(
+            Str->getBeginLoc(),
+            diag::warn_initializer_string_for_char_array_too_long_no_nonstring)
+            << ArrayLen << StrLength << Str->getSourceRange();
 
-          return nullptr;
-        };
-        if (const ValueDecl *D = FindCorrectEntity(&Entity);
-            !D || !D->hasAttr<NonStringAttr>())
-          S.Diag(
-              Str->getBeginLoc(),
-              diag::
-                  warn_initializer_string_for_char_array_too_long_no_nonstring)
-              << ArrayLen << StrLength << Str->getSourceRange();
-      }
       // Always emit the C++ compatibility diagnostic.
       S.Diag(Str->getBeginLoc(),
              diag::warn_initializer_string_for_char_array_too_long_for_cpp)