add some timing diagnostics

pass --enable-timing-info to print a nice table like this:

```
                Name       Start         End    Duration     Percent
          Initialize      0.0000      0.0000      0.0000      0.0001
   Semantic Analysis      0.0000      0.0421      0.0420      0.2109
     Code Generation      0.0421      0.0620      0.0200      0.1003
    LLVM Emit Object      0.0620      0.1852      0.1231      0.6180
  Build Dependencies      0.1852      0.1974      0.0122      0.0615
           LLVM Link      0.1974      0.1993      0.0018      0.0093
         Generate .h      0.1993      0.1993      0.0000      0.0000
               Total      0.0000      0.1993      0.1993      1.0000
```

1 files changed, 30 insertions, 0 deletions

diff --git a/src/codegen.cpp b/src/codegen.cpp
index 01b763d5b0..12cff60a17 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -57,6 +57,9 @@ PackageTableEntry *new_package(const char *root_src_dir, const char *root_src_pa
 
 CodeGen *codegen_create(Buf *root_source_dir, const ZigTarget *target) {
     CodeGen *g = allocate<CodeGen>(1);
+
+    codegen_add_time_event(g, "Initialize");
+
     g->import_table.init(32);
     g->builtin_fn_table.init(32);
     g->primitive_type_table.init(32);
@@ -3654,6 +3657,8 @@ static LLVMValueRef build_alloca(CodeGen *g, TypeTableEntry *type_entry, const c
 static void do_code_gen(CodeGen *g) {
     assert(!g->errors.length);
 
+    codegen_add_time_event(g, "Code Generation");
+
     delete_unused_builtin_fns(g);
     generate_error_name_table(g);
     generate_enum_name_tables(g);
@@ -3977,6 +3982,8 @@ static void do_code_gen(CodeGen *g) {
     LLVMVerifyModule(g->module, LLVMAbortProcessAction, &error);
 #endif
 
+    codegen_add_time_event(g, "LLVM Emit Object");
+
     char *err_msg = nullptr;
     Buf *out_file_o = buf_create_from_buf(g->root_out_name);
     const char *o_ext = target_o_file_ext(&g->zig_target);
@@ -4730,6 +4737,8 @@ static PackageTableEntry *create_zigrt_pkg(CodeGen *g) {
 }
 
 void codegen_add_root_code(CodeGen *g, Buf *src_dir, Buf *src_basename, Buf *source_code) {
+    codegen_add_time_event(g, "Semantic Analysis");
+
     Buf source_path = BUF_INIT;
     os_path_join(src_dir, src_basename, &source_path);
 
@@ -5020,3 +5029,24 @@ void codegen_generate_h_file(CodeGen *g) {
     if (fclose(out_h))
         zig_panic("unable to close h file: %s", strerror(errno));
 }
+
+void codegen_print_timing_report(CodeGen *g, FILE *f) {
+    double start_time = g->timing_events.at(0).time;
+    double end_time = g->timing_events.last().time;
+    double total = end_time - start_time;
+    fprintf(f, "%20s%12s%12s%12s%12s\n", "Name", "Start", "End", "Duration", "Percent");
+    for (size_t i = 0; i < g->timing_events.length - 1; i += 1) {
+        TimeEvent *te = &g->timing_events.at(i);
+        TimeEvent *next_te = &g->timing_events.at(i + 1);
+        fprintf(f, "%20s%12.4f%12.4f%12.4f%12.4f\n", te->name,
+                te->time - start_time,
+                next_te->time - start_time,
+                next_te->time - te->time,
+                (next_te->time - te->time) / total);
+    }
+    fprintf(f, "%20s%12.4f%12.4f%12.4f%12.4f\n", "Total", 0.0, total, total, 1.0);
+}
+
+void codegen_add_time_event(CodeGen *g, const char *name) {
+    g->timing_events.append({os_get_time(), name});
+}