C++ Interop: Add support for char (#5988)

Added tests for different character types.
`char` is currently not in primitives prelude, so had to use full
prelude.

C++ Interop Demo:

```carbon
// main.carbon

library "Main";

import Cpp inline '''
auto output_char(char c) -> void {
  printf("%c", c);
}
''';

fn Run() -> i32 {
  let msg: array(Core.Char, 13) =
      ('H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!', '\n');
  for (c: Core.Char in msg) {
    Cpp.output_char(c);
  }
  return 0;
}
```

```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
Hello world!
```

Part of #5263.

Author: bricknerb

toolchain/check/import_cpp.cpp

@@ -1158,6 +1158,35 @@ static auto MakeIntType(Context& context, IntId size_id, bool is_signed)
   return ExprAsType(context, Parse::NodeId::None, type_inst_id);
 }
 
+// Maps a C++ builtin integer type to a Carbon type.
+// TODO: Handle integer types that map to named aliases.
+static auto MapBuiltinIntegerType(Context& context, SemIR::LocId loc_id,
+                                  clang::QualType qual_type,
+                                  const clang::BuiltinType& type) -> TypeExpr {
+  clang::ASTContext& ast_context = context.ast_context();
+  unsigned width = ast_context.getIntWidth(qual_type);
+  bool is_signed = type.isSignedInteger();
+  auto int_n_type = ast_context.getIntTypeForBitwidth(width, is_signed);
+  if (ast_context.hasSameType(qual_type, int_n_type)) {
+    TypeExpr type_expr =
+        MakeIntType(context, context.ints().Add(width), is_signed);
+    // Try to make sure integer types of 32 or 64 bits are complete so we can
+    // check against them when deciding whether we need to generate a thunk.
+    if (width == 32 || width == 64) {
+      SemIR::TypeId type_id = type_expr.type_id;
+      if (!context.types().IsComplete(type_id)) {
+        TryToCompleteType(context, type_id, loc_id);
+      }
+    }
+    return type_expr;
+  }
+  if (ast_context.hasSameType(qual_type, ast_context.CharTy)) {
+    return ExprAsType(context, Parse::NodeId::None,
+                      MakeCharTypeLiteral(context, Parse::NodeId::None));
+  }
+  return TypeExpr::None;
+}
+
 // Maps a C++ builtin type to a Carbon type.
 // TODO: Support more builtin types.
 static auto MapBuiltinType(Context& context, SemIR::LocId loc_id,
@@ -1171,25 +1200,9 @@ static auto MapBuiltinType(Context& context, SemIR::LocId loc_id,
                           context, SemIR::BoolType::TypeInstId)));
   }
   if (type.isInteger()) {
-    unsigned width = context.ast_context().getIntWidth(qual_type);
-    bool is_signed = type.isSignedInteger();
-    auto int_n_type =
-        context.ast_context().getIntTypeForBitwidth(width, is_signed);
-    if (context.ast_context().hasSameType(qual_type, int_n_type)) {
-      TypeExpr type_expr =
-          MakeIntType(context, context.ints().Add(width), is_signed);
-      // Try to make sure integer types of 32 or 64 bits are complete so we can
-      // check against them when deciding whether we need to generate a thunk.
-      if (width == 32 || width == 64) {
-        SemIR::TypeId type_id = type_expr.type_id;
-        if (!context.types().IsComplete(type_id)) {
-          TryToCompleteType(context, type_id, loc_id);
-        }
-      }
-      return type_expr;
-    }
-    // TODO: Handle integer types that map to named aliases.
-  } else if (type.isFloatingPoint()) {
+    return MapBuiltinIntegerType(context, loc_id, qual_type, type);
+  }
+  if (type.isFloatingPoint()) {
     if (type.isFloat16Type() || type.isFloat32Type() || type.isDoubleType() ||
         type.isFloat128Type()) {
       return ExprAsType(

toolchain/check/literal.cpp

@@ -25,6 +25,11 @@ auto MakeIntLiteral(Context& context, Parse::NodeId node_id, IntId int_id)
        .int_id = int_id});
 }
 
+auto MakeCharTypeLiteral(Context& context, Parse::NodeId node_id)
+    -> SemIR::InstId {
+  return LookupNameInCore(context, node_id, "Char");
+}
+
 auto MakeIntTypeLiteral(Context& context, Parse::NodeId node_id,
                         SemIR::IntKind int_kind, IntId size_id)
     -> SemIR::InstId {

toolchain/check/literal.h

@@ -18,6 +18,10 @@ namespace Carbon::Check {
 auto MakeIntLiteral(Context& context, Parse::NodeId node_id, IntId int_id)
     -> SemIR::InstId;
 
+// Forms a char type expression for `char` literal.
+auto MakeCharTypeLiteral(Context& context, Parse::NodeId node_id)
+    -> SemIR::InstId;
+
 // Forms an integer type expression for either an `iN` or `uN` literal.
 auto MakeIntTypeLiteral(Context& context, Parse::NodeId node_id,
                         SemIR::IntKind int_kind, IntId size_id)

toolchain/check/testdata/interop/cpp/class/field.carbon

@@ -2,7 +2,7 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
-// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run: