[WebAssembly] Don't generate empty type section.

lld/trunk/test/wasm/section-symbol-relocs.yaml

 # RELOC-NEXT:         Index:           0
 # RELOC-NEXT:         Offset:          0x00000007
 # RELOC-NEXT:         Addend:          6
 # RELOC-NEXT:       - Type:            R_WASM_SECTION_OFFSET_I32
 # RELOC-NEXT:         Index:           1
 # RELOC-NEXT:         Offset:          0x0000000B
 # RELOC-NEXT:         Addend:          3
 # RELOC-NEXT:    Name:            green
+#
+# RELOC:        SymbolTable:
+# RELOC-NEXT:     - Index:           0
+# RELOC-NEXT:       Kind:            SECTION
+# RELOC-NEXT:       Flags:           [ BINDING_LOCAL ]
+# RELOC-NEXT:       Section:         2
+# RELOC-NEXT:     - Index:           1
+# RELOC-NEXT:       Kind:            SECTION
+# RELOC-NEXT:       Flags:           [ BINDING_LOCAL ]
+# RELOC-NEXT:       Section:         3

lld/trunk/wasm/Writer.cpp

for (const Symbol *Sym : GOTSymbols) {
     else
       Import.Module = "GOT.func";
     Import.Field = Sym->getName();
     writeImport(OS, Import);
   }
 }
 
 void Writer::createTypeSection() {
+  if (!Types.size())
+    return;
   SyntheticSection *Section = createSyntheticSection(WASM_SEC_TYPE);
   raw_ostream &OS = Section->getStream();
   writeUleb128(OS, Types.size(), "type count");
   for (const WasmSignature *Sig : Types)
     writeSig(OS, *Sig);
 }
 
 void Writer::createFunctionSection() {