Consolidate default Clang argument handling (#6545)

This unifies the default Clang arguments between the `clang` subcommand,
the `link` subcommand, and the `ClangInvocation` built for C++ interop.

This sets the stage to integrate either pre-built or on-demand runtimes
flags for both of these. However, this PR should have very little
practical difference. The biggest functional change is wrapping the
default arguments in flags to allow unused flags so that we can build a
collection of flags viable across compile and link.

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

@@ -4,6 +4,8 @@
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/none.carbon
 //
+// EXTRA-ARGS: --target=aarch64-unknown-linux
+//
 // AUTOUPDATE
 // TIP: To test this file alone, run:
 // TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/cpp_namespace.carbon