[lld][WebAssembly] Fix for debug relocations against undefined function symbols

[sbc100]

Summary

This is very similar to https://reviews.llvm.org/D103557 but applies to
symbols which are undefined at link time rather than compile time.

We already have code that handles symbols which were defined at link
time but dead stripped by --gc-sections (See
test/wasm/debug-removed-fn.ll). In that case the symbols are not live
(!isLive()). However, we can also have live symbols (which are
references by the program) but which are undefined at link time and are
imported by the linker.

In the test case here the symbol undef is used but is not defined
in the program but is imported by the linker due to the
--import-undefined flag.

Fixes: https://github.com/emscripten-core/emscripten/issues/15528

lld/test/wasm/debug-undefined-fs.s

+# Verify that we can handle R_WASM_FUNCTION_OFFSET relocations against live but
+# undefined symbols.  Test that the .debug_info and .debug_int sections are
+# generated without error
+#
+# Based on llvm/test/MC/WebAssembly/debuginfo-relocs.s
+#
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld --import-undefined %t.o -o %t.wasm
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+.functype undef () -> ()
+
+bar:
+    .functype bar () -> ()
+    end_function
+
+    .globl _start
+_start:
+    .functype _start () -> ()
+    call bar
+    call undef
+    end_function
+
+.section .debug_int,"",@
+.Ld:
+  .int32 1
+.size .Ld, 4
+
+.section .debug_info,"",@
+    .int32 bar
+    .int32 undef
+    .int32 .Ld
+
+# CHECK:          Name:            .debug_info
+# CHECK-NEXT:     Payload:         02000000FFFFFFFF00000000
+# CHECK:          Name:            .debug_int
+# CHECK-NEXT:     Payload:         '01000000'

lld/wasm/InputFiles.cpp

uint64_t ObjFile::calcNewValue(const WasmRelocation &reloc, uint64_t tombstone,
   case R_WASM_GLOBAL_INDEX_I32:
     if (auto gs = dyn_cast<GlobalSymbol>(sym))
       return gs->getGlobalIndex();
     return sym->getGOTIndex();
   case R_WASM_TAG_INDEX_LEB:
     return getTagSymbol(reloc.Index)->getTagIndex();
   case R_WASM_FUNCTION_OFFSET_I32:
   case R_WASM_FUNCTION_OFFSET_I64: {
+    if (isa<UndefinedFunction>(sym)) {
+      return tombstone ? tombstone : reloc.Addend;
+    }
     auto *f = cast<DefinedFunction>(sym);
     return f->function->getOffset(f->function->getFunctionCodeOffset() +
                                   reloc.Addend);
   }
   case R_WASM_SECTION_OFFSET_I32:
     return getSectionSymbol(reloc.Index)->section->getOffset(reloc.Addend);
   case R_WASM_TABLE_NUMBER_LEB:
     return getTableSymbol(reloc.Index)->getTableNumber();