Encapsulate clang::ASTUnit in SemIR::CppFile. (#6459)

This intends to avoid proliferation of dependencies on the exact API of
`clang::ASTUnit`, and would enable us to more easily switch to a
different approach that gives us more control over the construction of
the Clang AST.

Also remove some unnecessary tracking of the `CppFile` and instead
always retrieve it from the `SemIR::File`.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/cpp/location.cpp

@@ -56,7 +56,7 @@ static auto GetFileInfo(Context& context, SemIR::CheckIRId ir_id) -> FileInfo {
 
 auto GetCppLocation(Context& context, SemIR::LocId loc_id)
     -> clang::SourceLocation {
-  if (!context.sem_ir().clang_ast_unit()) {
+  if (!context.sem_ir().cpp_file()) {
     return clang::SourceLocation();
   }