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_llvm/llvm-wrapper/RustWrapper.cpp

@@ -1,4 +1,5 @@
 #include "LLVMWrapper.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DiagnosticHandler.h"
 #include "llvm/IR/DiagnosticInfo.h"
@@ -116,6 +117,11 @@ extern "C" void LLVMRustPrintPassTimings() {
   TimerGroup::printAll(OS);
 }
 
+extern "C" void LLVMRustPrintStatistics() {
+  raw_fd_ostream OS(2, false); // stderr.
+  llvm::PrintStatistics(OS);
+}
+
 extern "C" LLVMValueRef LLVMRustGetNamedValue(LLVMModuleRef M, const char *Name,
                                               size_t NameLen) {
   return wrap(unwrap(M)->getNamedValue(StringRef(Name, NameLen)));