[vim] Pass correct -std flags to YCM libclang.

TBR=verwaest@chromium.org

Review URL: https://codereview.chromium.org/915933003

Cr-Commit-Position: refs/heads/master@{#26566}

tools/vim/v8.ycm_extra_conf.py

@@ -46,7 +46,7 @@ import sys
 # Flags from YCM's default config.
 flags = [
 '-DUSE_CLANG_COMPLETER',
-'-std=c++11',
+'-std=gnu++0x',
 '-x',
 'c++',
 ]