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/language_server/context.cpp

@@ -10,6 +10,7 @@
 
 #include "common/check.h"
 #include "common/raw_string_ostream.h"
+#include "llvm/TargetParser/Host.h"
 #include "toolchain/base/shared_value_stores.h"
 #include "toolchain/check/check.h"
 #include "toolchain/diagnostics/diagnostic.h"
@@ -152,7 +153,8 @@ auto Context::File::SetText(Context& context, std::optional<int64_t> version,
   // TODO: Include the prelude.
   Check::CheckParseTrees(
       units, llvm::ArrayRef<Parse::GetTreeAndSubtreesFn>(getter),
-      /*prelude_import=*/false, fs, context.vlog_stream(), /*fuzzing=*/false);
+      /*prelude_import=*/false, fs, llvm::sys::getDefaultTargetTriple(),
+      context.vlog_stream(), /*fuzzing=*/false);
 
   // Note we need to publish diagnostics even when empty.
   // TODO: Consider caching previously published diagnostics and only publishing