[clang][modules] Separate parsing of modulemaps (#119740)
This separates out parsing of modulemaps from updating the
`clang::ModuleMap` information.
Currently this has no effect other than slightly changing diagnostics.
Upcoming changes will use this to allow searching for modules without
fully processing modulemaps.
This creates a new `modulemap` namespace because there are too many
things called ModuleMap* right now that mean different things. I'd like
to clean this up, but I'm not sure yet what I want to call everything.
This also drops the `SourceLocation` from `moduleMapFileRead`. This is
never used in tree, and in future patches I plan to make the modulemap
parser use a different `SourceManager` so that we can share modulemap
parsing between `CompilerInstance`s. This will make the `SourceLocation`
meaningless.
