Factor out a Check::CppContext holding C++-specific check state (#6482)

* Move `Sema` access from `CppFile` into `CppContext`.
* Move the mangle context from `SemIR::File` into `CppContext`.
* Move source location mapping state from `Context` into `CppContext`.

Also factor out the `GenerateAst` function that builds the `CppContext`
and `CppFile` into its own file.

toolchain/check/cpp/location.cpp

@@ -33,7 +33,7 @@ static auto GetFileInfo(Context& context, SemIR::CheckIRId ir_id) -> FileInfo {
   }
 
   // If we've seen this file before, reuse the same FileID.
-  auto& file_start_locs = context.cpp_carbon_file_locations();
+  auto& file_start_locs = context.cpp_context()->carbon_file_locations();
   if (static_cast<int>(file_start_locs.size()) <= file_index) {
     // Never valid; prepare a slot for the caching below.
     file_start_locs.resize(file_index + 1);