llvm-symbolizer: fix handling of DW_AT_specification in FRAME.

Summary:
Use getSubroutineName() to the the subrouting name; this function knows
how to handle cases when DW_TAG_subprogram refers to an earlier
declaration:

0x00000050:     DW_TAG_subprogram
                  DW_AT_linkage_name    ("_ZN1A1fEv")
                  DW_AT_name    ("f")
...
0x00000067:   DW_TAG_subprogram
                DW_AT_low_pc    (0x0000000000000000)
                DW_AT_high_pc   (0x0000000000000020)
                DW_AT_specification     (0x00000050 "_ZN1A1fEv")
...
0x0000008c:     DW_TAG_variable

Reviewers: pcc, vitalybuka, jdoerfert

Subscribers: srhines, hiraditya, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70630