[clang] [llvm] [RISCV][MC] Add support for Q extension (PR #139369)

[Pengcheng Wang via cfe-commits]

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@@ -291,6 +291,13 @@ def HasStdExtD : Predicate<"Subtarget->hasStdExtD()">,
                  AssemblerPredicate<(all_of FeatureStdExtD),
                                     "'D' (Double-Precision Floating-Point)">;
 
+def FeatureStdExtQ
+    : RISCVExtension<2, 2, "Quad-Precision Floating-Point", [FeatureStdExtD]>,
+      RISCVExtensionBitmask<0, 16>;
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wangpc-pp wrote:

Where does the `16` come from? I don't see it in https://github.com/riscv-non-isa/riscv-c-api-doc/blob/main/src/c-api.adoc#extension-bitmask-definitions. I think we should remove the `RISCVExtensionBitmask` for now.

https://github.com/llvm/llvm-project/pull/139369