Move TLS check from LValueExprEvaluator::VisitVarDecl to

Move TLS check from LValueExprEvaluator::VisitVarDecl to
CheckLValueConstantExpression.

Richard pointed out that using the address of a TLS variable is ok in a
core C++11 constant expression, as long as it isn't part of the eventual
result of constant expression evaluation. Having the check in
CheckLValueConstantExpression accomplishes this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162850 91177308-0d34-0410-b5e6-96231b3b80d8