Add a flag to build a single ASTContext shared across all compilations (#7567)

Instead of building one Clang `ASTContext` per compilation, the
`--share-cpp-ast` flag causes us to build a single `ASTContext` and
share it across all contexts. One new abstraction is added: `CppDomain`
represents the Carbon-side view of a Clang AST that might be shared
across multiple `SemIR::File`s. This object owns the Clang instance and
the AST.

For now, we have no isolation between the C++ state exposed to different
Carbon compilations, and we have no multiplexing of generated LLVM IR
from C++ into different Carbon compilations, so the mode is not usable
yet. The plan is to keep it behind a flag until it's ready.

Assisted-by: Gemini via Antigravity

toolchain/check/testdata/interop/cpp/basics/inline/modules.carbon

@@ -21,7 +21,7 @@ import Cpp inline '''
 // CHECK:STDERR:    13 | export module Foo;
 // CHECK:STDERR:       |        ^
 // CHECK:STDERR: <carbon Cpp imports>:1:1: note: add 'module;' to the start of the file to introduce a global module fragment [CppInteropParseNote]
-// CHECK:STDERR:     1 | # 6 "fail_export_module_in_inline_cpp.carbon"
+// CHECK:STDERR:     1 | ;
 // CHECK:STDERR:       | ^
 // CHECK:STDERR:
 export module Foo;
@@ -37,7 +37,7 @@ import Cpp inline '''
 // CHECK:STDERR:    17 | module Foo;
 // CHECK:STDERR:       | ^
 // CHECK:STDERR: <carbon Cpp imports>:1:1: note: add 'module;' to the start of the file to introduce a global module fragment [CppInteropParseNote]
-// CHECK:STDERR:     1 | # 6 "fail_module_in_inline_cpp.carbon"
+// CHECK:STDERR:     1 | ;
 // CHECK:STDERR:       | ^
 // CHECK:STDERR:
 // CHECK:STDERR: fail_module_in_inline_cpp.carbon:[[@LINE+4]]:8: error: module 'Foo' not found [CppInteropParseError]
@@ -65,7 +65,7 @@ int n;
 // CHECK:STDERR:    25 | module Foo;
 // CHECK:STDERR:       | ^
 // CHECK:STDERR: <carbon Cpp imports>:1:1: note: add 'module;' to the start of the file to introduce a global module fragment [CppInteropParseNote]
-// CHECK:STDERR:     1 | # 6 "fail_global_module_in_inline_cpp.carbon"
+// CHECK:STDERR:     1 | ;
 // CHECK:STDERR:       | ^
 // CHECK:STDERR:
 // CHECK:STDERR: fail_global_module_in_inline_cpp.carbon:[[@LINE+4]]:8: error: module 'Foo' not found [CppInteropParseError]

toolchain/check/testdata/interop/cpp/basics/share_ast.carbon

@@ -0,0 +1,64 @@
+// 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
+//
+// ARGS: compile --phase=check --share-cpp-ast %s
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/int.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/basics/share_ast.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/basics/share_ast.carbon
+
+// --- a.h
+#ifndef A_H
+#define A_H
+inline int GetA() { return 42; }
+#endif
+
+// --- b.h
+#ifndef B_H
+#define B_H
+#include "a.h"
+inline int GetB() { return 100; }
+#endif
+
+// --- a.carbon
+library "[[@TEST_NAME]]";
+import Cpp library "a.h";
+
+fn CallA() -> i32 {
+  return Cpp.GetA();
+}
+
+// --- todo_a_leaks.carbon
+library "[[@TEST_NAME]]";
+import Cpp;
+
+fn CallLeakedA() -> i32 {
+  // TODO: This should not be found as we have not imported "a.h".
+  return Cpp.GetA();
+}
+
+// --- a_again.carbon
+library "[[@TEST_NAME]]";
+import Cpp library "a.h";
+
+fn CallAAgain() -> i32 {
+  // This should be accepted as we have imported "a.h", even though we will just reuse the version from `a.carbon`.
+  return Cpp.GetA();
+}
+
+// --- b.carbon
+library "[[@TEST_NAME]]";
+import Cpp library "b.h";
+
+fn CallAIndirect() -> i32 {
+  return Cpp.GetA();
+}
+
+fn CallB() -> i32 {
+  return Cpp.GetB();
+}