Introduce ASTUnresolvedSet, an UnresolvedSet-like class, whose contents are

Introduce ASTUnresolvedSet, an UnresolvedSet-like class, whose contents are
allocated using the allocator associated with an ASTContext.

Use this inside CXXRecordDecl::DefinitionData instead of an UnresolvedSet to
avoid a potential memory leak.

rdar://12761275

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168771 91177308-0d34-0410-b5e6-96231b3b80d8