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/sem_ir/cpp_file.cpp

@@ -11,7 +11,7 @@
 
 namespace Carbon::SemIR {
 
-CppFile::CppFile(std::unique_ptr<clang::CompilerInstance> clang,
+CppFile::CppFile(std::shared_ptr<clang::CompilerInstance> clang,
                  llvm::LLVMContext* llvm_context)
     : clang_(std::move(clang)), llvm_context_(llvm_context) {}