Use partition_point in SourceMap::lookup_source_file_idx.

This makes it (a) a little simpler, and (b) more similar to
`SourceFile::lookup_line`.

compiler/rustc_span/src/source_map.rs

@@ -1072,11 +1072,7 @@ impl SourceMap {
     /// This index is guaranteed to be valid for the lifetime of this `SourceMap`,
     /// since `source_files` is a `MonotonicVec`
     pub fn lookup_source_file_idx(&self, pos: BytePos) -> usize {
-        self.files
-            .borrow()
-            .source_files
-            .binary_search_by_key(&pos, |key| key.start_pos)
-            .unwrap_or_else(|p| p - 1)
+        self.files.borrow().source_files.partition_point(|x| x.start_pos <= pos) - 1
     }
 
     pub fn count_lines(&self) -> usize {