compiler: fix few needless_pass_by_ref_mut clippy lints

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_session\src\config.rs:2013:16
     |
2013 |     early_dcx: &mut EarlyDiagCtxt,
     |                ^^^^^^^^^^^^^^^^^^ help: consider changing to: `&EarlyDiagCtxt`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_ast_passes\src\ast_validation.rs:1555:11
     |
1555 |     this: &mut AstValidator<'_>,
     |           ^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&AstValidator<'_>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
  --> compiler\rustc_infer\src\infer\snapshot\fudge.rs:16:12
   |
16 |     table: &mut UnificationTable<'_, 'tcx, T>,
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&UnificationTable<'_, 'tcx, T>`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_expand\src\expand.rs:961:13
    |
961 |     parser: &mut Parser<'a>,
    |             ^^^^^^^^^^^^^^^ help: consider changing to: `&Parser<'a>`
    |
    = warning: changing this function will impact semver compatibility
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index ff8165c4f7b..4bf0e51905f 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2010,7 +2010,7 @@ pub fn parse_crate_edition(early_dcx: &EarlyDiagCtxt, matches: &getopts::Matches
 }
 
 fn check_error_format_stability(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     unstable_opts: &UnstableOptions,
     error_format: ErrorOutputType,
 ) {
@@ -2574,7 +2574,7 @@ fn parse_remap_path_prefix(
 }
 
 fn parse_logical_env(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     matches: &getopts::Matches,
 ) -> FxIndexMap<String, String> {
     let mut vars = FxIndexMap::default();