[Tooling] Correct the "-std" compile command option.

[Tooling] Correct the "-std" compile command option.

Summary:
"-std c++11" is not valid in compiler, we have to use "-std=c++11".

Test in vscode with this patch, code completion for header works as expected.

Reviewers: sammccall

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D45512

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329786 91177308-0d34-0410-b5e6-96231b3b80d8