Add initial support for importing C++ constexprs as Carbon constants (#6770)

This allows a C++ constexpr to be used as an argument to a non-type
template parameter.

https://github.com/carbon-language/carbon-lang/issues/6717

toolchain/check/cpp/constant.cpp

@@ -0,0 +1,39 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/cpp/constant.h"
+
+#include "toolchain/check/cpp/import.h"
+#include "toolchain/check/eval.h"
+
+namespace Carbon::Check {
+
+// TODO: dedup with code in `MapConstant` and `TryEvaluateMacroToConstant`.
+auto MapAPValueToConstant(Context& context, SemIR::LocId loc_id,
+                          const clang::APValue& ap_value, clang::QualType type)
+    -> SemIR::ConstantId {
+  SemIR::TypeId type_id = ImportCppType(context, loc_id, type).type_id;
+  if (!type_id.has_value()) {
+    return SemIR::ConstantId::NotConstant;
+  }
+
+  if (ap_value.isInt()) {
+    if (type->isBooleanType()) {
+      auto value = SemIR::BoolValue::From(!ap_value.getInt().isZero());
+      return TryEvalInst(
+          context, SemIR::BoolLiteral{.type_id = type_id, .value = value});
+    } else {
+      CARBON_CHECK(type->isIntegralOrEnumerationType());
+
+      IntId int_id = context.ints().Add(ap_value.getInt());
+      return TryEvalInst(context,
+                         SemIR::IntValue{.type_id = type_id, .int_id = int_id});
+    }
+  } else {
+    // TODO: support other types.
+    return SemIR::ConstantId::NotConstant;
+  }
+}
+
+}  // namespace Carbon::Check