Add support for importing C++ inline functions (#5427)

This requires:
* Making `FunctionDecl` mutable since generating code
(`HandleTopLevelDecl()`) requires a mutable declaration and since we
manually add `used` attribute to force code generation.
* Passing the file system to `Lower` since it's needed by Clang code
generation.
* Creating an internal Clang LLVM module and link it against the Carbon
LLVM module.

Demo:

```c++
// hello_world.h

extern int puts;
inline void hello_world() {
  ((int (*)(const char*))&puts)("hello world");
}
```

```carbon
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  Cpp.hello_world();
  return 0;
}
```

```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
hello world
```

Based on https://github.com/carbon-language/carbon-lang/pull/5406.

Part of #5405.

toolchain/driver/compile_subcommand.cpp

@@ -712,9 +712,9 @@ auto CompilationUnit::RunLower() -> void {
     if (options_->include_debug_info) {
       subtrees = cache_->tree_and_subtrees_getters();
     }
-    module_ =
-        Lower::LowerToLLVM(*llvm_context_, subtrees, input_filename_, *sem_ir_,
-                           sem_ir_->cpp_ast(), &inst_namer, vlog_stream_);
+    module_ = Lower::LowerToLLVM(*llvm_context_, driver_env_->fs, subtrees,
+                                 input_filename_, *sem_ir_, sem_ir_->cpp_ast(),
+                                 &inst_namer, vlog_stream_);
   });
   if (vlog_stream_) {
     CARBON_VLOG("*** llvm::Module ***\n");
@@ -964,6 +964,7 @@ auto CompileSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   CARBON_VLOG_TO(driver_env.vlog_stream, "*** Check::CheckParseTrees ***\n");
   Check::CheckParseTrees(check_units, cache.tree_and_subtrees_getters(),
                          options_.prelude_import, driver_env.fs,
+                         options_.codegen_options.target,
                          driver_env.vlog_stream, driver_env.fuzzing);
   CARBON_VLOG_TO(driver_env.vlog_stream,
                  "*** Check::CheckParseTrees done ***\n");