[Bug c++/125329] Compilation performance of C++ reflection slower with `import std;` than with header inclusion

[boris at kolpackov dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125329

--- Comment #2 from Boris Kolpackov <boris at kolpackov dot net> ---
> Is that exactly the difference you'd get with otherwise empty TUs (besides the #include/import)?

No, with empty TUs, `import std;` becomes much faster:

          include-hdr  import-std  import-hdr
--------------------------------------------
GCC 16    2.82s        0.47s       0.57s


Feels like there has to be some lazy/on-demand name lookup for modules so that
if a name is not used by a TU, then it doesn't incur any overhead.