Remove premature caching of the global variables list in CompileUnit.
This is fixing a bug where
(lldb) target var g_ptr
would propulate the global variables list with exactly one entry because SymbolFileDWARF::ParseVariables() was invoked with a list of DIEs pre-filtered by name, such that a subsequent call to
(lldb) fr var --show-globals
would only list that one variable, because CompileUnit::m_variables was already initialized, fooling CompileUnit::GetVariableList().