Initial Reverse Interop implementation (#6901)

Add a clang::ExternalASTSource to begin exposing Carbon entities to
Clang - initially only a single `Carbon` top level namespace.

Subsequent work will add Carbon entities to this namespace.

Likely this CarbonExternalASTSource will be refactored into another
file, tie into/reference SemIR::File and CppFile, etc eventually - but
that'll wait for future patches.

If there's mechanical problems with the current implementation - how I'm
creating the new NamespaceDecl, etc - I'm all ears. It's very much in
the "it seems to work" state, not much more than that.

This does break Clang Modules (header modules, C++20 modules,
precompiled headers, etc) since they're implemented as an
ExternalASTSource as well, and Clang's ASTContext only supports one
ExternalASTSource at a time. To fix that regression we'll need to
implement some kind of ExternalASTSource multiplexing support - either
in Clang or Carbon (unclear which).

This regression of modules support can be observed by the following:
`A.h`
```
inline void f1() { }
```
`module.modulemap`
```
module A {
  header "A.h"
  export *
}
```
`test.carbon`
```
import Cpp inline '''
// Hardcode the pragma to ensure this isn't silently falling back to
// textual inclusion.
void f2() {
  f1();
}
''';
```
```
carbon compile test.carbon -- -I . -fmodules -fimplicit-modules -fmodules-cache-path=module_cache
```

I wrote a `file_test` test for this, but it doesn't /quite/ work because
`file_test` provides an in-memory filesystem for tests to make them more
hermetic, but Clang's Filesystem abstrtaction is for reading only - so
the module that's written out successfully can't be found when it needs
to be read back in - so the test doesn't pass as a baseline. Clang does
have support for `llvm::vfs::OutputBackend` which allows virtualizing
output - which I guess we could tie together with the InMemoryFilesystem
we use for input to make such a test work. But I guess that's not worth
the effort here?

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/testdata/interop/cpp/reverse/simple.carbon

@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/reverse/simple.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/reverse/simple.carbon
+
+// --- fail_simple.carbon
+
+library "[[@TEST_NAME]]";
+
+// Demonstrate that the 'Carbon' namespace has been injected into the C++ compilation.
+
+import Cpp inline '''
+// CHECK:STDERR: fail_simple.carbon:[[@LINE+4]]:9: error: no type named 'NonExistent' in namespace 'Carbon' [CppInteropParseError]
+// CHECK:STDERR:    11 | Carbon::NonExistent v;
+// CHECK:STDERR:       | ~~~~~~~~^
+// CHECK:STDERR:
+Carbon::NonExistent v;
+''';