Don't use guess_head_span in predicates_of for foreign span

Previously, the result of `predicates_of` for a foreign trait
would depend on the *current* state of the corresponding source
file in the foreign crate. This could lead to ICEs during incremental
compilation, since the on-disk contents of the upstream source file
could potentially change without the upstream crate being recompiled.

Additionally, this ensure that that the metadata we produce for a crate
only depends on its *compiled* upstream dependencies (e.g an rlib or
rmeta file), *not* the current on-disk state of the upstream crate
source files.

compiler/rustc_span/src/source_map.rs

@@ -567,6 +567,17 @@ impl SourceMap {
         }
     }
 
+    /// Returns whether or not this span points into a file
+    /// in the current crate. This may be `false` for spans
+    /// produced by a macro expansion, or for spans associated
+    /// with the definition of an item in a foreign crate
+    pub fn is_local_span(&self, sp: Span) -> bool {
+        let local_begin = self.lookup_byte_offset(sp.lo());
+        let local_end = self.lookup_byte_offset(sp.hi());
+        // This might be a weird span that covers multiple files
+        local_begin.sf.src.is_some() && local_end.sf.src.is_some()
+    }
+
     /// Returns the source snippet as `String` corresponding to the given `Span`.
     pub fn span_to_snippet(&self, sp: Span) -> Result<String, SpanSnippetError> {
         self.span_to_source(sp, |src, start_index, end_index| {