Add a flag to build a single ASTContext shared across all compilations (#7567)

Instead of building one Clang `ASTContext` per compilation, the
`--share-cpp-ast` flag causes us to build a single `ASTContext` and
share it across all contexts. One new abstraction is added: `CppDomain`
represents the Carbon-side view of a Clang AST that might be shared
across multiple `SemIR::File`s. This object owns the Clang instance and
the AST.

For now, we have no isolation between the C++ state exposed to different
Carbon compilations, and we have no multiplexing of generated LLVM IR
from C++ into different Carbon compilations, so the mode is not usable
yet. The plan is to keep it behind a flag until it's ready.

Assisted-by: Gemini via Antigravity

toolchain/check/cpp/context.cpp

@@ -11,18 +11,15 @@
 namespace Carbon::Check {
 
 CppContext::CppContext(clang::CompilerInstance& instance,
-                       std::unique_ptr<clang::Parser> parser)
+                       std::shared_ptr<clang::Parser> parser,
+                       std::unique_ptr<CppDiagnosticListener> listener)
     : ast_context_(&instance.getASTContext()),
       sema_(&instance.getSema()),
-      parser_(std::move(parser)) {}
+      parser_(std::move(parser)),
+      diagnostic_listener_(std::move(listener)) {}
 
 CppContext::~CppContext() = default;
 
-auto CppContext::set_diagnostic_listener(
-    std::unique_ptr<CppDiagnosticListener> listener) -> void {
-  diagnostic_listener_ = std::move(listener);
-}
-
 auto CppContext::clang_mangle_context() -> clang::MangleContext& {
   if (!clang_mangle_context_) {
     clang_mangle_context_.reset(ast_context().createMangleContext());