Add support for inline Cpp declarations. (#6994)

For #6830, add support for inline C++ fragments as a declaration rather
than as a packaging directive. For now, this uses `inline Cpp
<string-literal>;` as syntax. The prior `import Cpp inline
<string-literal>;` is left alone for the time being. We can decide
separately whether to remove that.

`inline Cpp` requires that there was at least one `import Cpp`. It's not
clear to me if that's the right design long-term, but it seems
reasonable for now.

Assisted-by: Gemini via Google Antigravity

toolchain/parse/handle_inline_decl.cpp

@@ -0,0 +1,43 @@
+// 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/lex/token_kind.h"
+#include "toolchain/parse/context.h"
+#include "toolchain/parse/handle.h"
+#include "toolchain/parse/node_kind.h"
+#include "toolchain/parse/state.h"
+
+namespace Carbon::Parse {
+
+auto HandleInlineDeclAfterIntroducer(Context& context) -> void {
+  auto state = context.PopState();
+
+  if (auto cpp_token = context.ConsumeIf(Lex::TokenKind::Cpp)) {
+    context.AddLeafNode(NodeKind::CppNameExpr, *cpp_token);
+  } else {
+    CARBON_DIAGNOSTIC(ExpectedCppAfterInline, Error,
+                      "expected `Cpp` after `inline`");
+    context.emitter().Emit(*context.position(), ExpectedCppAfterInline);
+    context.AddNode(NodeKind::InlineCppDecl,
+                    context.SkipPastLikelyEnd(state.token),
+                    /*has_error=*/true);
+    return;
+  }
+
+  if (auto str = context.ConsumeIf(Lex::TokenKind::StringLiteral)) {
+    context.AddLeafNode(NodeKind::InlineImportBody, *str);
+    context.AddNodeExpectingDeclSemi(state, NodeKind::InlineCppDecl,
+                                     Lex::TokenKind::Inline,
+                                     /*is_def_allowed=*/false);
+  } else {
+    CARBON_DIAGNOSTIC(ExpectedStringAfterInlineCpp, Error,
+                      "expected string literal after `inline Cpp`");
+    context.emitter().Emit(*context.position(), ExpectedStringAfterInlineCpp);
+    context.AddNode(NodeKind::InlineCppDecl,
+                    context.SkipPastLikelyEnd(state.token),
+                    /*has_error=*/true);
+  }
+}
+
+}  // namespace Carbon::Parse