Compile with C++17.

Switches from C++14 to C++17 standards.

Removes disabling the no-c++11-narrowing warning. We compile OK
with this warning enabled.

No code change.

Change-Id: Icb6479b0d9ef930b961f1ef965c18f22b6d847ff
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6000
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 38085b4..4c3f716 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -324,7 +324,7 @@
         '-fno-rtti',
         '-fdiagnostics-color',
     ])
-    cflags_cc.extend(['-std=c++14', '-Wno-c++11-narrowing'])
+    cflags_cc.extend(['-std=c++17'])
 
     if platform.is_linux():
       ldflags.append('-Wl,--as-needed')
@@ -378,6 +378,7 @@
         '/wd4996',
     ])
     cflags_cc.extend([
+        '/std:c++17',
         '/GR-',
         '/D_HAS_EXCEPTIONS=0',
     ])