[clang] [clang] Support -Wa, options -mmsa and -mno-msa (PR #99615)

[Fangrui Song via cfe-commits]

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@@ -2507,6 +2507,7 @@ static void CollectArgsForIntegratedAssembler(Compilation &C,
   bool Crel = false, ExperimentalCrel = false;
   bool UseRelaxRelocations = C.getDefaultToolChain().useRelaxRelocations();
   bool UseNoExecStack = false;
+  bool Msa = false;
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MaskRay wrote:

`bool Msa, NoMsa` encodes 4 states where `Msa&&NoMsa` is meaningless but not checked.

`std::optional<bool> Msa` encodes the desired 3 states:

* not set
* set and true
* set and false

https://github.com/llvm/llvm-project/pull/99615