Eliminate the uglified keyword __import_module__ for importing

modules. This leaves us without an explicit syntax for importing
modules in C/C++, because such a syntax needs to be discussed
first. In Objective-C/Objective-C++, the @import syntax is used to
import modules.

Note that, under -fmodules, C/C++ programs can import modules via the
#include mechanism when a module map is in place for that header. This
allows us to work with modules in C/C++ without committing to a syntax.

llvm-svn: 147467