[ELF] - Add testcase testing .gdb_index generation when base address of CU is used.

[grimar]

Summary

Currently lib/DebugInfo incorrectly handles the case when debug ranges
uses base address of CU. Section index is not calulcated properly and
LLD crashes on provided testcase.

D36097 and D37214 were posted to address the issue.
I tested this testcase with D37214.

Posting it for review, so we will be able to commit it when issue be fixed.

[ruiu]

LGTM

lld/trunk/test/ELF/gdb-index-base-addr.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: ld.lld --gdb-index %t1.o -o %t
+# RUN: llvm-dwarfdump -debug-dump=gdb_index %t | FileCheck %s
+
+# CHECK:      .gnu_index contents:
+# CHECK:       Address area offset = 0x28, has 2 entries:
+# CHECK-NEXT:  Low/High address = [0x201000, 0x201001) (Size: 0x1), CU id = 0
+# CHECK-NEXT:  Low/High address = [0x201003, 0x201006) (Size: 0x3), CU id = 0
+
+.text
+.globl foo
+.type foo,@function
+foo: 
+.Lfunc_begin0:
+  nop
+.Ltmp0:
+  nop
+  nop
+.Ltmp1:
+  nop
+  nop
+  nop
+.Ltmp2:
+
+.section .debug_abbrev,"",@progbits
+.byte 1                       # Abbreviation Code
+.byte 17                      # DW_TAG_compile_unit
+.byte 0                       # DW_CHILDREN_no
+.byte 37                      # DW_AT_producer
+.byte 14                      # DW_FORM_strp
+.byte 19                      # DW_AT_language
+.byte 5                       # DW_FORM_data2
+.byte 3                       # DW_AT_name
+.byte 14                      # DW_FORM_strp
+.byte 16                      # DW_AT_stmt_list
+.byte 23                      # DW_FORM_sec_offset
+.byte 27                      # DW_AT_comp_dir
+.byte 14                      # DW_FORM_strp
+.byte 17                      # DW_AT_low_pc
+.byte 1                       # DW_FORM_addr
+.byte 85                      # DW_AT_ranges
+.byte 23                      # DW_FORM_sec_offset
+.byte 0                       # EOM(1)
+.byte 0                       # EOM(2)
+.byte 0                       # EOM(3)
+
+.section .debug_info,"",@progbits
+.Lcu_begin0:
+.long 38                      # Length of Unit
+.short 4                      # DWARF version number
+.long .debug_abbrev           # Offset Into Abbrev. Section
+.byte 8                       # Address Size (in bytes)
+.byte 1                       # Abbrev [1] 0xb:0x1f DW_TAG_compile_unit
+.long 0                       # DW_AT_producer
+.short 4                      # DW_AT_language
+.long 0                       # DW_AT_name
+.long 0                       # DW_AT_stmt_list
+.long 0                       # DW_AT_comp_dir
+.quad .Lfunc_begin0           # DW_AT_low_pc
+.long .Ldebug_ranges0         # DW_AT_ranges
+
+.section .debug_ranges,"",@progbits
+.Ldebug_ranges0:
+ .quad .Lfunc_begin0-.Lfunc_begin0
+ .quad .Ltmp0-.Lfunc_begin0
+ .quad .Ltmp1-.Lfunc_begin0
+ .quad .Ltmp2-.Lfunc_begin0
+ .quad 0
+ .quad 0