[lld][WebAssembly] Fix for shared library symbols WRT replacing lazy symbols (llvm#124619)

The rule here, which I'm copying from the ELF linker, is that shared
library symbols should take presence, unless the symbol has already be
extracted from the archive. e.g:

```
$ wasm-ld foo.a foo.so ref.o  // .so wins
$ wasm-ld foo.a ref.o foo.so  // .a wins
```

In the first case the shared library takes precedence because the lazy
symbol is replaced by the .so symbol before it is extracted from the
archive. In the second example the ref.o file causes the archive to be
exracted before the .so file is processed, so in that case the archive
file wins.

Fixes: emscripten-core/emscripten#23501

Author: sbc100

lld/test/wasm/shared-lazy.s

@@ -0,0 +1,79 @@
+##  Based on lld/test/ELF/shared-lazy.s
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten a.s -o a.o
+# RUN: wasm-ld a.o --experimental-pic -shared -o a.so
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten b.s -o b.o
+# RUN: wasm-ld b.o --experimental-pic -shared -o b.so
+# RUN: llvm-ar rc a.a a.o
+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-emscripten ref.s -o ref.o
+# RUN: wasm-ld a.a b.so ref.o --experimental-pic -shared -o 1.so
+# RUN: obj2yaml 1.so | FileCheck %s
+# RUN: wasm-ld a.so a.a ref.o --experimental-pic -shared -o 1.so
+# RUN: obj2yaml 1.so | FileCheck %s
+
+## The definitions from a.so are used and we don't extract a member from the
+## archive.
+
+# CHECK:   - Type:            IMPORT
+# CHECK:            - Module:          GOT.mem
+# CHECK-NEXT:         Field:           x1
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         GlobalType:      I32
+# CHECK-NEXT:         GlobalMutable:   true
+# CHECK-NEXT:       - Module:          GOT.mem
+# CHECK-NEXT:         Field:           x2
+# CHECK-NEXT:         Kind:            GLOBAL
+# CHECK-NEXT:         GlobalType:      I32
+# CHECK-NEXT:         GlobalMutable:   true
+
+## The extracted x1 is defined as STB_GLOBAL.
+# RUN: wasm-ld ref.o a.a b.so -o 2.so --experimental-pic -shared
+# RUN: obj2yaml 2.so | FileCheck %s --check-prefix=CHECK2
+# RUN: wasm-ld a.a ref.o b.so -o 2.so --experimental-pic -shared
+# RUN: obj2yaml 2.so | FileCheck %s --check-prefix=CHECK2
+
+# CHECK2:       - Type:            EXPORT
+# CHECK2-NEXT:    Exports:
+# CHECK2-NEXT:      - Name:            __wasm_call_ctors
+# CHECK2-NEXT:        Kind:            FUNCTION
+# CHECK2-NEXT:        Index:
+# CHECK2-NEXT:      - Name:            x1
+# CHECK2-NEXT:        Kind:            GLOBAL
+# CHECK2-NEXT:        Index:
+# CHECK2-NEXT:      - Name:            x2
+# CHECK2-NEXT:        Kind:            GLOBAL
+
+#--- a.s
+.section .data.x1,"",@
+.global x1
+x1:
+  .byte 0
+.size x1, 1
+
+.section .data.x2,"",@
+.weak x2
+x2:
+  .byte 0
+.size x2, 1
+#--- b.s
+.section .data.x1,"",@
+.globl x1
+x1:
+  .byte 0
+.size x1, 1
+
+.section .data.x2,"",@
+.globl x2
+x2:
+  .byte 0
+.size x2, 1
+#--- ref.s
+.globl x1
+.globl x2
+.globl d
+.section .data.d,"",@
+d:
+  .int x1
+  .int x2
+.size d, 8

lld/wasm/SymbolTable.cpp

@@ -363,7 +363,7 @@ Symbol *SymbolTable::addSharedFunction(StringRef name, uint32_t flags,
     replaceSymbol<SharedFunctionSymbol>(sym, name, flags, file, sig);
   };
 
-  if (wasInserted) {
+  if (wasInserted || s->isLazy()) {
     replaceSym(s);
     return s;
   }
@@ -408,10 +408,18 @@ Symbol *SymbolTable::addSharedData(StringRef name, uint32_t flags,
   bool wasInserted;
   std::tie(s, wasInserted) = insert(name, file);
 
-  if (wasInserted || s->isUndefined()) {
+  if (wasInserted || s->isLazy()) {
     replaceSymbol<SharedData>(s, name, flags, file);
+    return s;
+  }
+
+  // Shared symbols should never replace locally-defined ones
+  if (s->isDefined()) {
+    return s;
   }
 
+  checkDataType(s, file);
+  replaceSymbol<SharedData>(s, name, flags, file);
   return s;
 }