[lld][WebAssembly] Allow relocations against non-live global symbols

[sbc100]

Summary

Just like the in case for function and data symbols this is needed to
support relocations in debug info sections which are allowed contains
relocations against non-live symbols.

The motivating use case is an object file that contains debug info that
references __stack_pointer (a local symbol) but does not actually
contain any uses of __stack_pointer.

Fixes: https://github.com/emscripten-core/emscripten/issues/14025

[sbc100]

Sadly I don't have test for this yet...

[sbc100]

Added test

[thakis]

this breaks tests: http://45.33.8.238/linux/45326/step_11.txt

lld/test/wasm/globals.s

1

due to this change, i suppose

[sbc100]

Sorry, landing fix now.

lld/test/wasm/debuginfo-undefined-global.s

+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld -o %t.wasm %t.o
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+# Debug sections are allowed to contains references to non-live symbols that
+# then get GC'd.  In this test the .debug_info seciton contains a reference to
+# foo which is not otherwise used and will not be marked a live in the output.
+# Verify the tombstone value is written to debug_info section.
+
+.globl  _start
+_start:
+  .functype _start () -> ()
+  end_function
+
+.section .debug_info,"",@
+  .int32 foo
+
+.globaltype foo, i32
+foo:
+
+# CHECK:       - Type:            CUSTOM
+# CHECK-NEXT:    Name:            .debug_info
+# CHECK-NEXT:    Payload:         FFFFFFFF

lld/test/wasm/globals.s

-# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+mm# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
 # RUN: wasm-ld %t.o -o %t.wasm
 # RUN: obj2yaml %t.wasm | FileCheck %s
 
 .globl _start
 .globl read_global
 .globl write_global
 
 .globaltype foo_global, i32

lld/wasm/InputFiles.cpp

uint64_t ObjFile::calcNewValue(const WasmRelocation &reloc, uint64_t tombstone,
   if (reloc.Type != R_WASM_TYPE_INDEX_LEB) {
     sym = symbols[reloc.Index];
 
     // We can end up with relocations against non-live symbols.  For example
     // in debug sections. We return a tombstone value in debug symbol sections
     // so this will not produce a valid range conflicting with ranges of actual
     // code. In other sections we return reloc.Addend.
 
-    if ((isa<FunctionSymbol>(sym) || isa<DataSymbol>(sym)) && !sym->isLive())
+    if (!isa<SectionSymbol>(sym) && !sym->isLive())
       return tombstone ? tombstone : reloc.Addend;
   }
 
   switch (reloc.Type) {
   case R_WASM_TABLE_INDEX_I32:
   case R_WASM_TABLE_INDEX_I64:
   case R_WASM_TABLE_INDEX_SLEB:
   case R_WASM_TABLE_INDEX_SLEB64: