[dwarf] Emit a DIGlobalVariable for constant strings.

An upcoming patch will extend llvm-symbolizer to provide the source line
information for global variables. The goal is to move AddressSanitizer
off of internal debug info for symbolization onto the DWARF standard
(and doing a clean-up in the process). Currently, ASan reports the line
information for constant strings if a memory safety bug happens around
them. We want to keep this behaviour, so we need to emit debuginfo for
these variables as well.

Reviewed By: dblaikie, rnk, aprantl

Differential Revision: https://reviews.llvm.org/D123534