[clang-tools-extra] [clang-tidy] fix cppcoreguidelines-narrowing-conversions false positives when narrowing integer to signed integer in C++20 (PR #116591)

[Congcong Cai via cfe-commits]

================
@@ -393,8 +393,14 @@ void NarrowingConversionsCheck::handleIntegralCast(const 
ASTContext &Context,
                                                    const Expr &Lhs,
                                                    const Expr &Rhs) {
   if (WarnOnIntegerNarrowingConversion) {
+    // From [conv.integral] since C++20
+    // The result is the unique value of the destination type that is congruent
+    // to the source integer modulo 2^N, where N is the width of the 
destination
+    // type.
+    if (getLangOpts().CPlusPlus20)
+      return;
----------------
HerrCai0907 wrote:

do you think we should apply similar approach to unsigned integer target type 
also to make the behaviour similar?
but it can be done separately

https://github.com/llvm/llvm-project/pull/116591
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