Handle module.modulemap.in for Clang module build of libc++

This generates modulemap file from module.modulemap.in like
https://github.com/llvm/llvm-project/pull/134699 does.

This also copies __config_site to build directory for modulemap update
too.

Bug: 412530604
Change-Id: I6e50cfee13f834deabed593d3f770d758b686caf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6475190
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1449776}