[lld][WebAssembly] Error on import of TLS symbols in shared libraries

[sbc100]

Summary

In https://reviews.llvm.org/D102044 we made exporting a TLS symbol
into an error, but we also want to error on import.

See https://github.com/emscripten-core/emscripten/issues/14461

[tlively]

Nice!

lld/test/wasm/tls-import.s

+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: not wasm-ld -shared --experimental-pic --shared-memory -o %t.wasm %t.o 2>&1 | FileCheck %s
+
+# CHECK: error: {{.*}}.o: TLS symbol is undefined, but TLS symbols cannot yet be imported: `foo`
+
+.globl _start
+_start:
+  .functype _start () -> ()
+  i32.const foo@TLSREL
+  drop
+  end_function
+
+.section  .custom_section.target_features,"",@
+  .int8 3
+  .int8 43
+  .int8 7
+  .ascii  "atomics"
+  .int8 43
+  .int8 11
+  .ascii  "bulk-memory"
+  .int8 43
+  .int8 15
+  .ascii "mutable-globals"

lld/wasm/Relocations.cpp

if (config->isPic) {
       case R_WASM_MEMORY_ADDR_SLEB64:
       case R_WASM_MEMORY_ADDR_LEB64:
         // Certain relocation types can't be used when building PIC output,
         // since they would require absolute symbol addresses at link time.
         error(toString(file) + ": relocation " + relocTypeToString(reloc.Type) +
               " cannot be used against symbol " + toString(*sym) +
               "; recompile with -fPIC");
         break;
+      case R_WASM_MEMORY_ADDR_TLS_SLEB:
+      case R_WASM_MEMORY_ADDR_TLS_SLEB64:
+        if (!sym->isDefined()) {
+          error(toString(file) +
+                ": TLS symbol is undefined, but TLS symbols cannot yet be "
+                "imported: `" +
+                toString(*sym) + "`");
+        }
+        break;
       case R_WASM_TABLE_INDEX_I32:
       case R_WASM_TABLE_INDEX_I64:
       case R_WASM_MEMORY_ADDR_I32:
       case R_WASM_MEMORY_ADDR_I64:
         // These relocation types are only present in the data section and
         // will be converted into code by `generateRelocationCode`.  This code
         // requires the symbols to have GOT entires.
         if (requiresGOTAccess(sym))