gn build: Windows: write PDBs when is_debug

[dmajor]

Summary

Without /DEBUG, the /Zi doesn't on its own create PDB files.

And since ninja runs multiple compilations in parallel, we need /FS to prevent contention on PDBs.

llvm/trunk/utils/gn/build/BUILD.gn

if (host_os != "win") {
     cflags += [ "-fdiagnostics-color" ]
     cflags_cc = [
       "-std=c++11",
       "-fno-exceptions",
       "-fvisibility-inlines-hidden",
     ]
   } else {
     if (is_debug) {
-      cflags += [ "/Zi" ]
+      cflags += [
+        "/Zi",
+        "/FS",
+      ]
+      ldflags += [ "/DEBUG" ]
     } else {
       cflags += [
         "/O2",
         "/Zc:inline",
       ]
     }
     defines += [
       "_CRT_SECURE_NO_DEPRECATE",
       "_CRT_SECURE_NO_WARNINGS",
       "_CRT_NONSTDC_NO_DEPRECATE",
       "_CRT_NONSTDC_NO_WARNINGS",
       "_SCL_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_WARNINGS",
 
       "_HAS_EXCEPTIONS=0",
       "_UNICODE",
       "UNICODE",
     ]
     cflags += [ "/EHs-c-" ]
 
     # The MSVC default value (1 MB) is not enough for parsing recursive C++
     # templates in Clang.
-    ldflags = [ "/STACK:10000000" ]
+    ldflags += [ "/STACK:10000000" ]
   }
 
   # Warning setup.
   if (host_os == "win" && !is_clang) {
     cflags += [
       # Suppress ''modifier' : used more than once' (__forceinline and inline).
       "-wd4141",