Implement system header lookup for interop import (#6128)

Using the toolchain-tasks suggested syntax of:
```
import Cpp header "<system_header.h>";
```

Author: dwblaikie

toolchain/check/cpp/import.cpp

@@ -103,10 +103,13 @@ static auto GenerateCppIncludesHeaderCode(
       GenerateLineMarker(context, code_stream,
                          context.tokens().GetLineNumber(
                              context.parse_tree().node_token(import.node_id)));
-      code_stream << "#include \""
-                  << FormatEscaped(
-                         context.string_literal_values().Get(import.library_id))
-                  << "\"\n";
+      auto name = context.string_literal_values().Get(import.library_id);
+      if (name.starts_with('<') && name.ends_with('>')) {
+        code_stream << "#include <"
+                    << FormatEscaped(name.drop_front().drop_back()) << ">\n";
+      } else {
+        code_stream << "#include \"" << FormatEscaped(name) << "\"\n";
+      }
     }
   }
 

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

@@ -79,6 +79,25 @@ impl library "[[@TEST_NAME]]";
 import Cpp library "header.h";
 //@dump-sem-ir-end
 
+
+// --- /usr/include/system_header.h
+
+auto system_function() -> void;
+
+// --- system_header.carbon
+
+library "[[@TEST_NAME]]";
+
+//@dump-sem-ir-begin
+import Cpp library "<system_header.h>";
+//@dump-sem-ir-end
+
+fn F() {
+//@dump-sem-ir-begin
+  Cpp.system_function();
+//@dump-sem-ir-end
+}
+
 // CHECK:STDOUT: --- cpp_in_inner_namespace.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -120,3 +139,36 @@ import Cpp library "header.h";
 // CHECK:STDOUT: imports {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- system_header.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %.98d: type = cpp_overload_set_type @system_function [concrete]
+// CHECK:STDOUT:   %empty_struct: %.98d = struct_value () [concrete]
+// CHECK:STDOUT:   %system_function.type: type = fn_type @system_function [concrete]
+// CHECK:STDOUT:   %system_function: %system_function.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Cpp: <namespace> = namespace file.%Cpp.import_cpp, [concrete] {
+// CHECK:STDOUT:     .system_function = %.769
+// CHECK:STDOUT:     import Cpp//...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %.769: %.98d = cpp_overload_set_value @system_function [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %system_function.decl: %system_function.type = fn_decl @system_function [concrete = constants.%system_function] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %Cpp.import_cpp = import_cpp {
+// CHECK:STDOUT:     import Cpp "<system_header.h>"
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
+// CHECK:STDOUT:   %system_function.ref: %.98d = name_ref system_function, imports.%.769 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %system_function.call: init %empty_tuple.type = call imports.%system_function.decl()
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT: }
+// CHECK:STDOUT: