[clang] [MS] Add /Zc:tlsGuards option to control tls guard emission (PR #113830)

[Eli Friedman via cfe-commits]

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@@ -4063,6 +4063,11 @@ defm threadsafe_statics : BoolFOption<"threadsafe-statics",
   NegFlag<SetFalse, [], [ClangOption, CC1Option],
           "Do not emit code to make initialization of local statics thread safe">,
   PosFlag<SetTrue>>;
+defm tls_guards : BoolFOption<"tls-guards",
+  LangOpts<"TlsGuards">, DefaultTrue,
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efriedma-quic wrote:

If we're going to expose this as a clang option (instead of just a clang-cc1 option), this needs a better name and description.  This specifically applies to the MS ABI rules for thread-local variables not defined inside a function definition.  Maybe call it `-fms-nonlocal-tls-guards`?  Not sure.

https://github.com/llvm/llvm-project/pull/113830