Correct newline emission in generated headers
Eliminates extraneous newlines between top-level Objective-C declarations in `-emit-objc-header` headers. Specifically, there should now always be exactly one—no more, no less—empty line between `@end` and whatever follows it.
Besides being more aesthetically pleasing, this eliminates ordering-dependent behavior where PrintAsClang would print an extra newline when visiting an empty extension, which meant that the order in which empty and non-empty extensions were visited during printing could result in whitespace differences in the compiler output. Printing the blank line is now conditional on whether `tell()` indicates that characters were actually written to the output.
Fixes rdar://143533893.
