DwarfAccelTable: Store the string symbol in the accelerator table to avoid duplicate lookup.

This also avoids the need for subtly side-effecting calls to manifest
strings in the string table at the point where items are added to the
accelerator tables.
R600: Fix function name printing in LowerCall

v2: Check both ExternalSymbol and GlobalAddress

Patch by: Jan Vesely <jan.vesely@rutgers.edu>