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/llvm/ffi.rs

@@ -1870,6 +1870,9 @@ extern "C" {
     /// Print the pass timings since static dtors aren't picking them up.
     pub fn LLVMRustPrintPassTimings();
 
+    /// Print the statistics since static dtors aren't picking them up.
+    pub fn LLVMRustPrintStatistics();
+
     pub fn LLVMStructCreateNamed(C: &Context, Name: *const c_char) -> &Type;
 
     pub fn LLVMStructSetBody<'a>(