Improve detection of global vs local variables

Summary:
Improve detection of global vs local variables.

Currently when a global variable is optimized out or otherwise has an unknown
location (DW_AT_location is empty) it gets reported as local.

I added two new heuristics:
 - if a mangled name is present, the variable is global (or static)
 - if DW_AT_location is present but invalid, the variable is global (or static)

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26908

llvm-svn: 287636