[clang][modules] Deprecate module.map in favor of module.modulemap (#75142)

This patch deprecates `module.map` in favor of `module.modulemap`, which
has been the preferred form since 2014. The eventual goal is to remove
support for `module.map` to reduce the number of stats Clang needs to do
while searching for module map files.

This patch touches a lot of files, but the majority of them are just
renaming tests or references to the file in comments or documentation.

The relevant files are:
* lib/Lex/HeaderSearch.cpp
* include/clang/Basic/DiagnosticGroups.td
* include/clang/Basic/DiagnosticLexKinds.td