[NFC] Refactor fast-math handling for clang driver (#81173)

This refactors the fast-math handling in the clang driver, moving the
settings into a lambda that is shared by the -ffp-model=fast and
-ffast-math code. Previously the -ffp-model=fast handler changed the
local option variable and fell through to the -ffast-math handler.

This refactoring is intended to prepare the way for decoupling the
-ffp-model=fast settings from the -ffast-math settings and possibly
introduce a less aggressive fp-model.

clang/lib/Driver/ToolChains/Clang.cpp

@@ -2778,6 +2778,26 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
   LangOptions::ComplexRangeKind Range = LangOptions::ComplexRangeKind::CX_None;
   std::string ComplexRangeStr = "";
 
+  // Lambda to set fast-math options. This is also used by -ffp-model=fast
+  auto applyFastMath = [&]() {
+    HonorINFs = false;
+    HonorNaNs = false;
+    MathErrno = false;
+    AssociativeMath = true;
+    ReciprocalMath = true;
+    ApproxFunc = true;
+    SignedZeros = false;
+    TrappingMath = false;
+    RoundingFPMath = false;
+    FPExceptionBehavior = "";
+    // If fast-math is set then set the fp-contract mode to fast.
+    FPContract = "fast";
+    // ffast-math enables limited range rules for complex multiplication and
+    // division.
+    Range = LangOptions::ComplexRangeKind::CX_Limited;
+    SeenUnsafeMathModeOption = true;
+  };
+
   if (const Arg *A = Args.getLastArg(options::OPT_flimited_precision_EQ)) {
     CmdArgs.push_back("-mlimit-float-precision");
     CmdArgs.push_back(A->getValue());
@@ -2842,9 +2862,8 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
             << Args.MakeArgString("-ffp-model=" + FPModel)
             << Args.MakeArgString("-ffp-model=" + Val);
       if (Val.equals("fast")) {
-        optID = options::OPT_ffast_math;
         FPModel = Val;
-        FPContract = "fast";
+        applyFastMath();
       } else if (Val.equals("precise")) {
         optID = options::OPT_ffp_contract;
         FPModel = Val;
@@ -3061,22 +3080,7 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
         continue;
       [[fallthrough]];
     case options::OPT_ffast_math: {
-      HonorINFs = false;
-      HonorNaNs = false;
-      MathErrno = false;
-      AssociativeMath = true;
-      ReciprocalMath = true;
-      ApproxFunc = true;
-      SignedZeros = false;
-      TrappingMath = false;
-      RoundingFPMath = false;
-      FPExceptionBehavior = "";
-      // If fast-math is set then set the fp-contract mode to fast.
-      FPContract = "fast";
-      SeenUnsafeMathModeOption = true;
-      // ffast-math enables fortran rules for complex multiplication and
-      // division.
-      Range = LangOptions::ComplexRangeKind::CX_Limited;
+      applyFastMath();
       break;
     }
     case options::OPT_fno_fast_math: