Split a cross-file Lower::Context out of Lower::FileContext. (#5583)

In preparation for lowering information from multiple `SemIR::File`s
into a single `llvm::Module`. The primary purpose of this is to support
lowering a local specific for an imported generic function, where the
instructions for the generic function are in a different file than the
instructions for the specific. See #5475 for a draft PR implementing
that functionality on top of this.

The per-`llvm::Module` state now lives in `Lower::Context`, and
`Lower::FileContext` tracks only the per-`SemIR::File` information.
`Lower::Context` should not mention any `SemIR` IDs that are
file-specific. For now, the C++ lowering and the specific coalescing
logic are kept per-file for simplicity.

toolchain/driver/compile_subcommand.cpp

@@ -713,8 +713,8 @@ auto CompilationUnit::RunLower() -> void {
       subtrees = cache_->tree_and_subtrees_getters();
     }
     module_ = Lower::LowerToLLVM(*llvm_context_, driver_env_->fs, subtrees,
-                                 input_filename_, *sem_ir_, sem_ir_->cpp_ast(),
-                                 &inst_namer, vlog_stream_);
+                                 input_filename_, *sem_ir_, &inst_namer,
+                                 vlog_stream_);
   });
   if (vlog_stream_) {
     CARBON_VLOG("*** llvm::Module ***\n");