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_codegen_llvm/src/lib.rs

@@ -181,6 +181,11 @@ impl WriteBackendMethods for LlvmCodegenBackend {
             llvm::LLVMRustPrintPassTimings();
         }
     }
+    fn print_statistics(&self) {
+        unsafe {
+            llvm::LLVMRustPrintStatistics();
+        }
+    }
     fn run_link(
         cgcx: &CodegenContext<Self>,
         diag_handler: &Handler,