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/check/node_stack.h

@@ -478,6 +478,8 @@ class NodeStack {
       case Parse::NodeKind::TuplePatternStart:
       case Parse::NodeKind::VariableInitializer:
       case Parse::NodeKind::VariableIntroducer:
+      case Parse::NodeKind::InlineImportBody:
+      case Parse::NodeKind::InlineIntroducer:
         return Id::Kind::None;
       case Parse::NodeKind::AdaptIntroducer:
       case Parse::NodeKind::AliasInitializer:
@@ -511,7 +513,6 @@ class NodeStack {
       case Parse::NodeKind::LibraryIntroducer:
       case Parse::NodeKind::LibrarySpecifier:
       case Parse::NodeKind::InlineImportSpecifier:
-      case Parse::NodeKind::InlineImportBody:
       case Parse::NodeKind::MatchCase:
       case Parse::NodeKind::MatchCaseGuard:
       case Parse::NodeKind::MatchCaseGuardIntroducer: