[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

llvm-svn: 329786