Remove (lots of) dead code

Found with https://github.com/est31/warnalyzer.

Dubious changes:
- Is anyone else using rustc_apfloat? I feel weird completely deleting
  x87 support.
- Maybe some of the dead code in rustc_data_structures, in case someone
  wants to use it in the future?
- Don't change rustc_serialize

  I plan to scrap most of the json module in the near future (see
  https://github.com/rust-lang/compiler-team/issues/418) and fixing the
  tests needed more work than I expected.

TODO: check if any of the comments on the deleted code should be kept.

compiler/rustc_session/src/config.rs

@@ -505,6 +505,7 @@ impl<'a> From<&'a ExternDepSpec> for rustc_lint_defs::ExternDepSpec {
 }
 
 impl Externs {
+    /// Used for testing.
     pub fn new(data: BTreeMap<String, ExternEntry>) -> Externs {
         Externs(data)
     }
@@ -604,13 +605,6 @@ impl Input {
         }
     }
 
-    pub fn get_input(&mut self) -> Option<&mut String> {
-        match *self {
-            Input::File(_) => None,
-            Input::Str { ref mut input, .. } => Some(input),
-        }
-    }
-
     pub fn source_name(&self) -> FileName {
         match *self {
             Input::File(ref ifile) => ifile.clone().into(),
@@ -778,12 +772,6 @@ impl Options {
             || self.debugging_opts.query_dep_graph
     }
 
-    #[inline(always)]
-    pub fn enable_dep_node_debug_strs(&self) -> bool {
-        cfg!(debug_assertions)
-            && (self.debugging_opts.query_dep_graph || self.debugging_opts.incremental_info)
-    }
-
     pub fn file_path_mapping(&self) -> FilePathMapping {
         FilePathMapping::new(self.remap_path_prefix.clone())
     }
@@ -1060,9 +1048,6 @@ mod opt {
     pub fn flag_s(a: S, b: S, c: S) -> R {
         stable(longer(a, b), move |opts| opts.optflag(a, b, c))
     }
-    pub fn flagopt_s(a: S, b: S, c: S, d: S) -> R {
-        stable(longer(a, b), move |opts| opts.optflagopt(a, b, c, d))
-    }
     pub fn flagmulti_s(a: S, b: S, c: S) -> R {
         stable(longer(a, b), move |opts| opts.optflagmulti(a, b, c))
     }
@@ -1073,15 +1058,6 @@ mod opt {
     pub fn multi(a: S, b: S, c: S, d: S) -> R {
         unstable(longer(a, b), move |opts| opts.optmulti(a, b, c, d))
     }
-    pub fn flag(a: S, b: S, c: S) -> R {
-        unstable(longer(a, b), move |opts| opts.optflag(a, b, c))
-    }
-    pub fn flagopt(a: S, b: S, c: S, d: S) -> R {
-        unstable(longer(a, b), move |opts| opts.optflagopt(a, b, c, d))
-    }
-    pub fn flagmulti(a: S, b: S, c: S) -> R {
-        unstable(longer(a, b), move |opts| opts.optflagmulti(a, b, c))
-    }
 }
 
 /// Returns the "short" subset of the rustc command line options,
@@ -2459,7 +2435,7 @@ crate mod dep_tracking {
     }
 
     // This is a stable hash because BTreeMap is a sorted container
-    pub fn stable_hash(
+    crate fn stable_hash(
         sub_hashes: BTreeMap<&'static str, &dyn DepTrackingHash>,
         hasher: &mut DefaultHasher,
         error_format: ErrorOutputType,