Inspect DW_AT_const_value global static const variables

This patch adds support for printing global static const variables which are given a DW_AT_const_value DWARF tag by clang.

Fix for bug https://llvm.org/bugs/show_bug.cgi?id=25653

Reviewers: clayborg, tberghammer
Subscribers: emaste, lldb-commits
Differential Revision: http://reviews.llvm.org/D15576

llvm-svn: 255887