Auto merge of #48917 - petrochenkov:import, r=oli-obk

syntax: Make imports in AST closer to the source and cleanup their parsing

This is a continuation of https://github.com/rust-lang/rust/pull/45846 in some sense.

src/libsyntax/parse/lexer/mod.rs

@@ -214,7 +214,7 @@ impl<'a> StringReader<'a> {
 
         // Make the range zero-length if the span is invalid.
         if span.lo() > span.hi() || begin.fm.start_pos != end.fm.start_pos {
-            span = span.with_hi(span.lo());
+            span = span.shrink_to_lo();
         }
 
         let mut sr = StringReader::new_raw_internal(sess, begin.fm);