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).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302098 91177308-0d34-0410-b5e6-96231b3b80d8