Remember declaration scope qualifiers in the AST. Imposes no memory overhead

on unqualified declarations.

Patch by Enea Zaffanella!  Minimal adjustments:  allocate the ExtInfo nodes
with the ASTContext and delete them during Destroy().  I audited a bunch of
Destroy methods at the same time, to ensure that the correct teardown was
being done.

llvm-svn: 98540