Add builtins for basic float operations (#3897)

Adds support for unary `-` and binary `+`, `-`, `*`, `/` for floating
point types.

Real literals are now transformed to `llvm::APFloat`s during the check
phase into the `FloatLiteral` instruction.

This PR likely collides a bit with #3892 and might need to be updated
when that one is merged.

toolchain/sem_ir/builtin_function_kind.cpp

@@ -75,6 +75,17 @@ struct AnyInt {
   }
 };
 
+// Constraint that requires the type to be a float type.
+struct AnyFloat {
+  static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
+      -> bool {
+    if (BuiltinType<InstId::BuiltinFloatType>::Check(sem_ir, state, type_id)) {
+      return true;
+    }
+    return sem_ir.types().Is<FloatType>(type_id);
+  }
+};
+
 // Constraint that requires the type to be the type type.
 using Type = BuiltinType<InstId::BuiltinTypeType>;
 
@@ -139,6 +150,10 @@ using IntT = TypeParam<0, AnyInt>;
 // generic type parameter that is constrained to be an integer type.
 using IntU = TypeParam<1, AnyInt>;
 
+// Convenience name used in the builtin type signatures below for a first
+// generic type parameter that is constrained to be an float type.
+using FloatT = TypeParam<0, AnyFloat>;
+
 // Not a builtin function.
 constexpr BuiltinInfo None = {"", nullptr};
 
@@ -260,6 +275,26 @@ constexpr BuiltinInfo IntGreater = {"int.greater",
 constexpr BuiltinInfo IntGreaterEq = {
     "int.greater_eq", ValidateSignature<auto(IntT, IntT)->Bool>};
 
+// "float.negate": float negation.
+constexpr BuiltinInfo FloatNegate = {"float.negate",
+                                     ValidateSignature<auto(FloatT)->FloatT>};
+
+// "float.add": float addition.
+constexpr BuiltinInfo FloatAdd = {
+    "float.add", ValidateSignature<auto(FloatT, FloatT)->FloatT>};
+
+// "float.sub": float subtraction.
+constexpr BuiltinInfo FloatSub = {
+    "float.sub", ValidateSignature<auto(FloatT, FloatT)->FloatT>};
+
+// "float.mul": float multiplication.
+constexpr BuiltinInfo FloatMul = {
+    "float.mul", ValidateSignature<auto(FloatT, FloatT)->FloatT>};
+
+// "float.div": float division.
+constexpr BuiltinInfo FloatDiv = {
+    "float.div", ValidateSignature<auto(FloatT, FloatT)->FloatT>};
+
 }  // namespace BuiltinFunctionInfo
 
 CARBON_DEFINE_ENUM_CLASS_NAMES(BuiltinFunctionKind) = {