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.

llvm-svn: 162850