rustc_llvm: Add a -Z print-llvm-stats option to expose LLVM statistics.

LLVM has a neat [statistics] feature that tracks how often optimizations kick
in. It's very handy for optimization work. Since we expose the LLVM pass
timings, I thought it made sense to expose the LLVM statistics too.

[statistics]: https://llvm.org/docs/ProgrammersManual.html#the-statistic-class-stats-option

compiler/rustc_session/src/options.rs

@@ -1672,6 +1672,9 @@ options! {
         "make rustc print the total optimization fuel used by a crate"),
     print_llvm_passes: bool = (false, parse_bool, [UNTRACKED],
         "print the LLVM optimization passes being run (default: no)"),
+    #[rustc_lint_opt_deny_field_access("use `Session::print_llvm_stats` instead of this field")]
+    print_llvm_stats: bool = (true, parse_bool, [UNTRACKED],
+        "print LLVM statistics (default: no)"),
     print_mono_items: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "print the result of the monomorphization collection pass"),
     print_type_sizes: bool = (false, parse_bool, [UNTRACKED],

compiler/rustc_session/src/session.rs

@@ -1057,6 +1057,10 @@ impl Session {
         self.opts.unstable_opts.verbose
     }
 
+    pub fn print_llvm_stats(&self) -> bool {
+        self.opts.unstable_opts.print_llvm_stats
+    }
+
     pub fn verify_llvm_ir(&self) -> bool {
         self.opts.unstable_opts.verify_llvm_ir || option_env!("RUSTC_VERIFY_LLVM_IR").is_some()
     }