Modules: Teach the preprocessor to recognize 'import' only after an '@'.

The preprocessor currently recognizes module declarations to load a
module based on seeing the 'import' keyword followed by an
identifier. This sequence is fairly unlikely in C (one would need a
type named 'import'), but is more common in Objective-C (where a
variable named 'import' can cause problems). Since import declarations
currently require a leading '@', recognize that in the preprocessor as
well. Fixes <rdar://problem/15084587>.