Start converting pieces of DeclarationNameTable to be allocated using ASTContext's allocator.

While DeclarationNameTable doesn't leak, it uses 'malloc' too often.  Start with having
'CXXLiteralOperatorNames' allocated using ASTContext's allocator and add a 'DoDestroy()' method
to DeclarationNameTable that is called by ~ASTContext.

llvm-svn: 103426