[clang] [Clang] Add '-Warray-compare' flag for C++ below version 20 (PR #118031)

[Shafik Yaghmour via cfe-commits]

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@@ -15,15 +15,15 @@ void test(const char *d) {
   if (NULL == "/")
     return;
   if ("/" != DELIM) // expected-warning {{result of comparison against a string literal is unspecified (use an explicit string comparison function instead)}}
-    return;
+    return;         // cxx-warning at -1 {{comparison between two arrays}}
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shafik wrote:

This seems to overlap w/ `warn_stringcompare` we probably want to do some special handling here since string literals are a special case of arrays and they have their own comparison functions.

We can do this as part of the C++26 feature implementation. 

https://github.com/llvm/llvm-project/pull/118031