[Driver] Don't force .exe suffix for lld

[smeenai]

Summary

When cross-compiling to Windows using lld, we want the driver to invoke
it as lld-link rather than lld-link.exe. On Windows, the LLVM fs
functions take care of adding the .exe suffix where necessary, so we can
just drop the addition in the toolchain entirely.

[zturner]

Seems fine to me.

lib/Driver/ToolChains/MSVC.cpp

if (TC.getIsVS2017OrNewer() &&
           Environment.push_back(Args.MakeArgString(EnvVar));
         }
         Cursor += EnvVar.size() + 1 /*null-terminator*/;
       }
     }
   SkipSettingEnvironment:;
 #endif
   } else {
-    linkPath = Linker;
-    llvm::sys::path::replace_extension(linkPath, "exe");
-    linkPath = TC.GetProgramPath(linkPath.c_str());
+    linkPath = TC.GetProgramPath(Linker.str().c_str());
   }
 
   auto LinkCmd = llvm::make_unique<Command>(
       JA, *this, Args.MakeArgString(linkPath), CmdArgs, Inputs);
   if (!Environment.empty())
     LinkCmd->setEnvironment(Environment);
   C.addCommand(std::move(LinkCmd));
 }

test/Driver/cl-link.c

 // RUN: %clang_cl /Tc%s nonexistent.obj -### /link /libpath:somepath 2>&1 | FileCheck --check-prefix=NONEXISTENT %s
 // RUN: %clang_cl /Tc%s nonexistent.lib -### /link /libpath:somepath 2>&1 | FileCheck --check-prefix=NONEXISTENT %s
 // NONEXISTENT-NOT: no such file
 // NONEXISTENT: link.exe
 // NONEXISTENT: "/libpath:somepath"
 // NONEXISTENT: nonexistent
 
 // RUN: %clang_cl /Tc%s -fuse-ld=lld -### 2>&1 | FileCheck --check-prefix=USE_LLD %s
-// USE_LLD: lld-link.exe
+// USE_LLD: lld-link