asm goto: fix out-of-bounds read of Constraints after rC362045

When parsing goto labels, Names and Exprs are expanded but Constraints
is not, this may cause a out-of-bounds read later in:

// GCCAsmStmt::GCCAsmStmt
// `constraints` has only `NumExprs - NumLabels` elements
  Constraints = new (C) StringLiteral*[NumExprs];
  std::copy(constraints, constraints + NumExprs, Constraints);

llvm-svn: 362067