[lld] Fix SEGFAULT in wasm-ld when importing wrapped symbol (PR #169656)

[Lukas Döllerer via llvm-commits]

================
@@ -0,0 +1,36 @@
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %s -o %t.o
+# RUN: wasm-ld -emit-relocs -wrap nosuchsym -wrap foo -allow-undefined -o %t.wasm %t.o
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+.globl foo
+.globl _start
+
+foo:
+  .functype foo () -> (i32)
+  i32.const 1
+  end_function
+
+_start:
+  .functype _start () -> ()
+  call  foo
+  drop
+  end_function
+
+# CHECK:      - Type:            IMPORT
+# CHECK-NEXT:   Imports:
+# CHECK-NEXT:     - Module:          env
+# CHECK-NEXT:       Field:           __wrap_foo
+# CHECK-NEXT:       Kind:            FUNCTION
+# CHECK-NEXT        SigIndex:        0
+
+# CHECK:      - Type:            CODE
+# CHECK-NEXT:   Relocations:
+# CHECK-NEXT:     - Type:            R_WASM_FUNCTION_INDEX_LEB
+# CHECK-NEXT:       Index:           1
----------------
Lukasdoe wrote:

I just stepped through the code a bit and I think this is intended behavior. 
```
  - Type:            CODE
    Relocations:
      - Type:            R_WASM_FUNCTION_INDEX_LEB
        Index:           1
        Offset:          0x4
    Functions:
      - Index:           1
        Locals:          []
        Body:            1080808080000B
  - Type:            CUSTOM
    Name:            linking
    Version:         2
    SymbolTable:
      - Index:           0
        Kind:            FUNCTION
        Name:            _start
        Flags:           [  ]
        Function:        1
      - Index:           1
        Kind:            FUNCTION
        Name:            __wrap_foo
        Flags:           [ UNDEFINED ]
        Function:        0
  - Type:            CUSTOM
    Name:            name
    FunctionNames:
      - Index:           0
        Name:            __wrap_foo
      - Index:           1
        Name:            _start
```
The symbol index is a different one than the function index. `__wrap_foo` has symbol index 1, which is why the relocation is emitted for index 1, while `_start` has function index 1, explaining why this function is shown.

https://github.com/llvm/llvm-project/pull/169656