[WebAssembly] Honor --allow-undefined even for explicit exports

[sbc100]

Summary

When a symbol is exported via --export=foo but --allow-undefined
is also specified, the symbol is now allowed to be undefined.
Previously we were special casing such symbols.

This combinations of behavior is exactly what emescripten
requires. Although we are trying hard not to allow emscripten
specific features in lld, this one makes sense.

Enforce this behavior by added this case to test/wasm/undefined.ll.

[ruiu]

LGTM

[sbc100]

• remove extra line

test/wasm/undefined.ll

 ; RUN: llc -filetype=obj %s -o %t.o
 ; RUN: wasm-ld --check-signatures --allow-undefined -o %t.wasm %t.o
 
 ; Fails due to undefined 'foo' and also 'baz'
 ; RUN: not wasm-ld --check-signatures --undefined=baz -o %t.wasm %t.o 2>&1 | FileCheck %s
 ; CHECK: error: {{.*}}.o: undefined symbol: foo
 ; CHECK: error: undefined symbol: baz
 
-; But succeeds if we pass a file containing 'foo' as --allow-undefined-file.
+; Succeeds if we pass a file containing 'foo' as --allow-undefined-file.
 ; RUN: echo 'foo' > %t.txt
 ; RUN: wasm-ld --check-signatures --allow-undefined-file=%t.txt -o %t.wasm %t.o
 
+; Succeeds even if a missing symbol is added via --export
+; RUN: echo 'foo' > %t.txt
+; RUN: wasm-ld --check-signatures --allow-undefined --export=xxx -o %t.wasm %t.o
+
 target triple = "wasm32-unknown-unknown-wasm"
 
 ; Takes the address of the external foo() resulting in undefined external
 @bar = hidden local_unnamed_addr global i8* bitcast (i32 ()* @foo to i8*), align 4
 
 declare i32 @foo() #0
 
 define hidden void @_start() local_unnamed_addr #0 {
 entry:
     ret void
 }

wasm/Driver.cpp

if (errorCount())
     return;
 
   // Handle --export.
   for (auto *Arg : Args.filtered(OPT_export)) {
     StringRef Name = Arg->getValue();
     Symbol *Sym = Symtab->find(Name);
     if (Sym && Sym->isDefined())
       Sym->setHidden(false);
-    else
+    else if (!Config->AllowUndefined)
       error("symbol exported via --export not found: " + Name);
   }
 
   if (EntrySym)
     EntrySym->setHidden(false);
 
   if (errorCount())
     return;
 
   // Do size optimizations: garbage collection
   markLive();
 
   // Write the result to the file.
   writeResult();
 }