Remove premature caching of the global variables list in CompileUnit.

This fixes a bug where

  (lldb) target var g_ptr

would populate 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().

CompileUnit::GetVariableList() grabs the *complete* list of variables
via (SymbolFileDWARF, ...)::ParseVariablesForContext and that still
calls CompileUnit::SetVariableList(variables) which acts as the
caching mechanism.

Differential Revision: https://reviews.llvm.org/D46220

llvm-svn: 331230