[clang] [ARM] fix "+fp.dp" in multilib selection (PR #67412)

[Dominik Wójt via cfe-commits]

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@@ -420,20 +444,35 @@ bool ARM::appendArchExtFeatures(StringRef CPU, ARM::ArchKind AK,
     CPU = "generic";
 
   if (ArchExt == "fp" || ArchExt == "fp.dp") {
+    const ARM::FPUKind DefaultFPU = getDefaultFPU(CPU, AK);
     ARM::FPUKind FPUKind;
     if (ArchExt == "fp.dp") {
+      const bool IsDP = ArgFPUKind != ARM::FK_INVALID &&
+                        ArgFPUKind != ARM::FK_NONE &&
+                        isDoublePrecision(getFPURestriction(ArgFPUKind));
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domin144 wrote:

The `appendArchExtFeatures` is called repeatedly in `DecodeARMFeatures`. There can be multiple FPU related extensions specified. e.g. `-march=armv8-m.main+nofp+fp+nofp.dp` should disable FPU, enable default FPU(fpv5-d16) and then switch to single precission equivalent FPU(fpv5-sp-d16).

https://github.com/llvm/llvm-project/pull/67412