[clang] Consistently use "load" to refer to populating clang::ModuleMap (#132970)
Now that we have ModuleMapFile.cpp which parses module maps, it's
confusing what ModuleMap::parseModuleMapFile actually does. HeaderSearch
already called this loading a module map, so consistently use that term
in ModuleMap too.
An upcoming patch will allow just parsing a module map without loading
the modules from it.