[ELF] --gdb-index: support --icf={safe,all}

The combination has not been tested before. In the case of ICF,
`e.section->getVA(0)` equals the start address of the output section.

This can cause incorrect overlapping with the actual function at the
start of the output section and potentially trigger a GDB internal error
in `dw2_find_pc_sect_compunit_symtab` (presumably because:
if a short address range incorrectly starts at the start address of the
output section, GDB may pick it instead of the correct longer address
range. When mapping an address within the long address range but
out of the scope of the short address range, the routine may find
nothing - while the code asserts that it can find something).

Note that in the case of ICF there may be duplicate address range entries,
but GDB appears to be fine with them.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D89751

Author: MaskRay

lld/ELF/SyntheticSections.cpp

@@ -2914,7 +2914,8 @@ void GdbIndexSection::writeTo(uint8_t *buf) {
   uint32_t cuOff = 0;
   for (GdbChunk &chunk : chunks) {
     for (AddressEntry &e : chunk.addressAreas) {
-      uint64_t baseAddr = e.section->getVA(0);
+      // In the case of ICF there may be duplicate address range entries.
+      const uint64_t baseAddr = e.section->repl->getVA(0);
       write64le(buf, baseAddr + e.lowAddress);
       write64le(buf + 8, baseAddr + e.highAddress);
       write32le(buf + 16, e.cuIndex + cuOff);

lld/test/ELF/gdb-index-icf.s

@@ -0,0 +1,67 @@
+# REQUIRES: x86
+## Test that the address range contributed by an ICF folded function is identical
+## to that of the folded-in function. Not considering ICF may lead to an address
+## range whose low address equals the start address of the output section.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld --gdb-index --icf=all -Ttext=0x1000 %t.o -o %t
+# RUN: llvm-dwarfdump --gdb-index %t | FileCheck %s
+
+# CHECK:      Address area offset = 0x38, has 2 entries:
+# CHECK-NEXT:   Low/High address = [0x1001, 0x1002) (Size: 0x1), CU id = 0
+# CHECK-NEXT:   Low/High address = [0x1001, 0x1002) (Size: 0x1), CU id = 1
+
+.text
+nop
+
+.section .text.0,"ax"
+.Lfunc_begin0:
+  ret
+.Lfunc_end0:
+
+.section .text.1,"ax"
+.Lfunc_begin1:
+  ret
+.Lfunc_end1:
+
+.section  .debug_abbrev,"",@progbits
+  .byte  1                         # Abbreviation Code
+  .byte  65                        # DW_TAG_type_unit
+  .byte  0                         # DW_CHILDREN_no
+  .byte  0                         # EOM(1)
+  .byte  0                         # EOM(2)
+
+  .byte  2                         # Abbreviation Code
+  .byte  17                        # DW_TAG_compile_unit
+  .byte  0                         # DW_CHILDREN_no
+  .byte  17                        # DW_AT_low_pc
+  .byte  1                         # DW_FORM_addr
+  .byte  18                        # DW_AT_high_pc
+  .byte  6                         # DW_FORM_data4
+  .byte  0                         # EOM(1)
+  .byte  0                         # EOM(2)
+
+  .byte  0                         # EOM(3)
+
+.section  .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Lcu_end0-.Lcu_begin0-4    # Length of Unit
+  .short 5                         # DWARF version number
+  .byte  1                         # DWARF Unit Type
+  .byte  8                         # Address Size
+  .long  .debug_abbrev             # Offset Into Abbrev. Section
+  .byte  2                         # Abbrev [2] DW_TAG_compile_unit
+  .quad  .Lfunc_begin0             # DW_AT_low_pc
+  .long  .Lfunc_end0-.Lfunc_begin0 # DW_AT_high_pc
+.Lcu_end0:
+
+.Lcu_begin1:
+  .long .Lcu_end1-.Lcu_begin1-4    # Length of Unit
+  .short 5                         # DWARF version number
+  .byte  1                         # DWARF Unit Type
+  .byte  8                         # Address Size
+  .long  .debug_abbrev             # Offset Into Abbrev. Section
+  .byte  2                         # Abbrev [2] DW_TAG_compile_unit
+  .quad  .Lfunc_begin1             # DW_AT_low_pc
+  .long  .Lfunc_end1-.Lfunc_begin1 # DW_AT_high_pc
+.Lcu_end1: