win/clang: Pass /X to the compiler.

This tells the compiler to not look in various env vars (e.g. %INCLUDE%)
which makes the build slightly more hermetic.

(MSVC still uses the env files to get at %INCLUDE% so we can't pass /X to cl,
but clang-cl gets all system include dirs passed as -imsvc flags instead.)

Also move /utf-8 from the warnings config to the compiler config as it's
not a warning flag.

Bug: 691286
Change-Id: I5306d33bbfacd03a267b9d1c082297a26aadf9b8
Reviewed-on: https://chromium-review.googlesource.com/993552Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548030}

build/config/compiler/BUILD.gn

@@ -247,6 +247,15 @@ config("compiler") {
         cflags += [ "-fstack-protector" ]
       }
     }
+  } else {
+    cflags += [
+      # Assume UTF-8 by default to avoid code page dependencies.
+      "/utf-8",
+    ]
+    if (is_clang) {
+      # Don't look for includes in %INCLUDE%.
+      cflags += [ "/X" ]
+    }
   }
 
   # Eliminate build metadata (__DATE__, __TIME__ and __TIMESTAMP__) for
@@ -1119,11 +1128,6 @@ config("default_warnings") {
       ldflags += [ "/WX" ]
     }
 
-    cflags += [
-      # Assume UTF-8 by default to avoid code page dependencies.
-      "/utf-8",
-    ]
-
     cflags += [
       # Warnings permanently disabled: