Instantiate C++ templates at end of file. (#6084)

Mark C++ functions as used when overload resolution selects them, and
trigger Clang's end-of-TU processing at the end of the Carbon
compilation to perform instantiation and other pending cleanup steps.

toolchain/check/testdata/interop/cpp/enum/anonymous.carbon

@@ -14,13 +14,13 @@
 
 enum { a, b, c };
 
-void F(decltype(a));
+void F(decltype(a)) {}
 
 class C {
  public:
   enum { d, e, f };
 
-  void F(decltype(d));
+  void F(decltype(d)) {}
 };
 
 // --- copy_enum.carbon

toolchain/check/testdata/interop/cpp/function/function.carbon

@@ -134,16 +134,21 @@ fn F() {
 
 static auto foo() -> void;
 
-// --- import_static.carbon
+// --- todo_fail_import_static.carbon
 
 library "[[@TEST_NAME]]";
 
+// TODO: Promote this warning to an error by default.
+// CHECK:STDERR: todo_fail_import_static.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: ./static.h:2:13: warning: function 'foo' has internal linkage but is not defined [CppInteropParseWarning]
+// CHECK:STDERR:     2 | static auto foo() -> void;
+// CHECK:STDERR:       |             ^
+// CHECK:STDERR:
 import Cpp library "static.h";
 
 fn F() {
   //@dump-sem-ir-begin
-  // TODO: We should eventually warn or error that `Cpp.foo` has internal
-  // linkage but is not defined.
+  // TODO: Produce a note pointing here.
   Cpp.foo();
   //@dump-sem-ir-end
 }
@@ -283,7 +288,7 @@ fn G() -> i32 {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: --- import_static.carbon
+// CHECK:STDOUT: --- todo_fail_import_static.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]

toolchain/check/testdata/interop/cpp/function/inline.carbon

@@ -42,11 +42,17 @@ inline void foo();
 
 library "[[@TEST_NAME]]";
 
+// TODO: Promote this warning to an error by default.
+// CHECK:STDERR: todo_fail_import_without_definition.carbon:[[@LINE+5]]:10: in file included here [InCppInclude]
+// CHECK:STDERR: ./without_definition.h:2:13: warning: inline function 'foo' is not defined [CppInteropParseWarning]
+// CHECK:STDERR:     2 | inline void foo();
+// CHECK:STDERR:       |             ^
+// CHECK:STDERR:
 import Cpp library "without_definition.h";
 
 fn MyF() {
   //@dump-sem-ir-begin
-  // TODO: Error on using an inline function without definition.
+  // TODO: Produce a note pointing here.
   Cpp.foo();
 
   // Don't error on repeated calls.
@@ -126,12 +132,12 @@ fn MyF() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @MyF() {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %Cpp.ref.loc9: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
-// CHECK:STDOUT:   %foo.ref.loc9: %.c5d = name_ref foo, imports.%.a21 [concrete = constants.%empty_struct]
-// CHECK:STDOUT:   %foo.call.loc9: init %empty_tuple.type = call imports.%foo.decl()
-// CHECK:STDOUT:   %Cpp.ref.loc12: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
-// CHECK:STDOUT:   %foo.ref.loc12: %.c5d = name_ref foo, imports.%.a21 [concrete = constants.%empty_struct]
-// CHECK:STDOUT:   %foo.call.loc12: init %empty_tuple.type = call imports.%foo.decl()
+// CHECK:STDOUT:   %Cpp.ref.loc15: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref.loc15: %.c5d = name_ref foo, imports.%.a21 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %foo.call.loc15: init %empty_tuple.type = call imports.%foo.decl()
+// CHECK:STDOUT:   %Cpp.ref.loc18: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %foo.ref.loc18: %.c5d = name_ref foo, imports.%.a21 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %foo.call.loc18: init %empty_tuple.type = call imports.%foo.decl()
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:

toolchain/check/testdata/interop/cpp/namespace.carbon

@@ -94,7 +94,7 @@ fn MyF() {
 
 // --- inline.h
 
-inline namespace { namespace N { inline namespace { void foo(); } } }
+inline namespace { namespace N { inline namespace { void foo() {} } } }
 
 // --- import_inline.carbon