[llvm-link] Improve link time for bitcode archives [NFC]

Linking large bitcode archives currently takes a lot of time with llvm-link,
this patch adds couple improvements which reduce link time for archives
- Use one Linker instance for archive instead of recreating it for each member
- Lazy load archive members

Reviewed By: tra, jdoerfert

Differential Revision: https://reviews.llvm.org/D94643

Author: sndmitriev

llvm/tools/llvm-link/llvm-link.cpp

@@ -153,6 +153,7 @@ static std::unique_ptr<Module> loadArFile(const char *Argv0,
   Error Err = Error::success();
   object::Archive Archive(*Buffer, Err);
   ExitOnErr(std::move(Err));
+  Linker L(*Result);
   for (const object::Archive::Child &C : Archive.children(Err)) {
     Expected<StringRef> Ename = C.getName();
     if (Error E = Ename.takeError()) {
@@ -186,7 +187,12 @@ static std::unique_ptr<Module> loadArFile(const char *Argv0,
       return nullptr;
     }
 
-    std::unique_ptr<Module> M = parseIR(MemBuf.get(), ParseErr, Context);
+    std::unique_ptr<Module> M;
+    if (DisableLazyLoad)
+      M = parseIR(MemBuf.get(), ParseErr, Context);
+    else
+      M = getLazyIRModule(MemoryBuffer::getMemBuffer(MemBuf.get(), false),
+                          ParseErr, Context);
 
     if (!M.get()) {
       errs() << Argv0 << ": ";
@@ -197,7 +203,7 @@ static std::unique_ptr<Module> loadArFile(const char *Argv0,
     }
     if (Verbose)
       errs() << "Linking member '" << ChildName << "' of archive library.\n";
-    if (Linker::linkModules(*Result, std::move(M)))
+    if (L.linkInModule(std::move(M)))
       return nullptr;
   } // end for each child
   ExitOnErr(std::move(Err));