DebugInfo: Change DIBuilder to make distinct DIGlobalVariables

A long overdue change to make DIGlobalVariable distinct.  Much like
DISubprogram definitions (changed in r246098), it isn't logical to
unique DIGlobalVariable definitions from two different compile units.

(Longer-term, we should also find a way to reverse the link between
GlobalVariable and DIGlobalVariable, and between DIGlobalVariable and
DICompileUnit, so that debug info to do with optimized-out globals
disappears.  Admittedly it's harder than with Function/DISubprogram,
since global variables may be constant-folded and the debug info should
still describe that somehow.)

llvm-svn: 267301