Add #pragma clang module begin/end pragmas and generate them when preprocessing a module.
These pragmas are intended to simulate the effect of entering or leaving a file
with an associated module. This is not completely implemented yet: declarations
between the pragmas will not be attributed to the correct module, but macro
visibility is already functional.
Modules named by #pragma clang module begin must already be known to clang (in
some module map that's either loaded or on the search path).