[clang] Turn `-Wdeprecated-literal-operator` on by default (PR #111027)

[Aaron Ballman via cfe-commits]

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@@ -503,17 +503,16 @@ bool Sema::checkLiteralOperatorId(const CXXScopeSpec &SS,
     const IdentifierInfo *II = Name.Identifier;
     ReservedIdentifierStatus Status = II->isReserved(PP.getLangOpts());
     SourceLocation Loc = Name.getEndLoc();
-    if (!PP.getSourceManager().isInSystemHeader(Loc)) {
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AaronBallman wrote:

I don't think we need it any longer because I believe we suppress diagnostics in system headers via the diagnostics engine itself; you can test it out by using GNU line markers to pretend some code is a system header: https://godbolt.org/z/PsaM19Pe8

https://github.com/llvm/llvm-project/pull/111027