Heap-allocate the attribute vectors in

Heap-allocate the attribute vectors in
ASTContext::DeclAttrs. Otherwise, iterators will go stale when the
DenseMap reallocates, which can cause crashes when, e.g., looping over
the attributes in a template to instantiate them and add the results
to the instantiation of that template.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@112488 91177308-0d34-0410-b5e6-96231b3b80d8