[LLD][COFF] Support GNU style == aliases

D46245 added support for this in llvm-libtool, but while lld-link can
also create .lib files from .def files it didn't support aliases.

I compared the Inputs/library.def test against the output from
llvm-libtool and it matches, except for the fact that lld-link reorders
functions for some reason.

I have also verified that this fixes a bug I was running into while
trying to compile .def files to .lib files in MinGW-w64 (using lld-link
instead of llvm-libtool).

Differential Revision: https://reviews.llvm.org/D113365

lld/COFF/Config.h

@@ -43,6 +43,7 @@ static const auto I386 = llvm::COFF::IMAGE_FILE_MACHINE_I386;
 struct Export {
   StringRef name;       // N in /export:N or /export:E=N
   StringRef extName;    // E in /export:E=N
+  StringRef aliasTarget; // GNU specific: N in "alias == N"
   Symbol *sym = nullptr;
   uint16_t ordinal = 0;
   bool noname = false;
@@ -63,6 +64,7 @@ struct Export {
 
   bool operator==(const Export &e) {
     return (name == e.name && extName == e.extName &&
+            aliasTarget == e.aliasTarget &&
             ordinal == e.ordinal && noname == e.noname &&
             data == e.data && isPrivate == e.isPrivate);
   }

lld/COFF/Driver.cpp

@@ -810,6 +810,7 @@ static void createImportLibrary(bool asLib) {
     e2.Name = std::string(e1.name);
     e2.SymbolName = std::string(e1.symbolName);
     e2.ExtName = std::string(e1.extName);
+    e2.AliasTarget = std::string(e1.aliasTarget);
     e2.Ordinal = e1.ordinal;
     e2.Noname = e1.noname;
     e2.Data = e1.data;
@@ -908,6 +909,7 @@ static void parseModuleDefs(StringRef path) {
     }
     e2.name = saver.save(e1.Name);
     e2.extName = saver.save(e1.ExtName);
+    e2.aliasTarget = saver.save(e1.AliasTarget);
     e2.ordinal = e1.Ordinal;
     e2.noname = e1.Noname;
     e2.data = e1.Data;

lld/test/COFF/Inputs/library.def

@@ -1,5 +1,6 @@
 LIBRARY library
 EXPORTS
   function
+  alias == function
   data DATA
   constant CONSTANT

lld/test/COFF/lib.test

@@ -1,6 +1,16 @@
 # RUN: lld-link /machine:x64 /def:%S/Inputs/library.def /out:%t.lib
 # RUN: llvm-nm %t.lib | FileCheck %s
 
+CHECK: 00000000 a @comp.id
+CHECK: 00000000 a @feat.00
+CHECK: 00000000 W alias
+CHECK:          U function
+
+CHECK: 00000000 a @comp.id
+CHECK: 00000000 a @feat.00
+CHECK: 00000000 W __imp_alias
+CHECK:          U __imp_function
+
 CHECK: 00000000 R __imp_constant
 CHECK: 00000000 R constant