gn: fix debug symbols on Windows

Separate PDB generation from debug/release so that PDB is generated also
for release builds.

Thanks to ivberg@ for the inspiration behind the patch!

Fixes: https://github.com/google/perfetto/issues/631
Change-Id: I3a59fd03f4a9ce4996528239f34cacb4008321df
diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index 19c8029..34f9ae1 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -319,9 +319,11 @@
     include_dirs = win_msvc_inc_dirs  # Defined in msvc.gni.
   }
 
+  if (is_win) {
+    cflags += [ "/Zi" ]
+  }
   if (is_debug) {
     if (is_win) {
-      cflags += [ "/Z7" ]
       if (is_clang) {
         # Required to see symbols in windbg when building with clang-cl.exe.
         cflags += [ "-gcodeview-ghash" ]
@@ -394,7 +396,7 @@
   }
 }
 
-config("debug_symbols") {
+config("debug_noopt") {
   cflags = []
   if (is_win) {
     cflags = [ "/Od" ]