cmake no longer needs to deal with -std=c++11 checks.

LLVM requires and handles this now and has the correct compiler
version checks. This block of code for cmake in LLDB is no longer
needed.

Summary: cmake no longer needs to deal with -std=c++11 checks.

Reviewers: labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10994

llvm-svn: 241590