[WebAssembly] Remove custom handling for undefined entry. NFC

[sbc100]

Summary

Handle undefined entry just like any other symbols.

This code was needed back when we were not able to write
out the synthetic symbol for main.

Add tests to make sure we can handle this now.

[ruiu]

LGTM

[ncw]

LGTM

test/wasm/undefined-entry.test

 RUN: llc -filetype=obj -mtriple=wasm32-unknown-unknown-wasm %p/Inputs/ret32.ll -o %t.ret32.o
 RUN: not lld -flavor wasm -o %t.wasm %t.ret32.o 2>&1 | FileCheck %s
 
 CHECK: error: undefined symbol: _start
+
+RUN: not lld -flavor wasm -entry=foo -o %t.wasm %t.ret32.o 2>&1 | FileCheck %s -check-prefix=CHECK-CUSTOM
+
+CHECK-CUSTOM: error: undefined symbol: foo
+
+RUN: lld -flavor wasm -entry=foo --allow-undefined -o %t.wasm %t.ret32.o

wasm/Driver.cpp

for (auto *Arg : Args.filtered(OPT_export)) {
     Symbol *Sym = Symtab->find(Arg->getValue());
     if (!Sym || !Sym->isDefined())
       error("symbol exported via --export not found: " +
             Twine(Arg->getValue()));
     else
       Sym->setHidden(false);
   }
 
-  if (!Config->Entry.empty() && !Symtab->find(Config->Entry)->isDefined())
-    error("entry point not found: " + Config->Entry);
   if (errorCount())
     return;
 
   // Write the result to the file.
   writeResult();
 }