Unconditionally pass -lto_library, remove -Wliblto warning.

[thakis]

Summary

https://reviews.llvm.org/D25932 made it so that clang always checks if libLTO.dylib is present on disk, even if -flto is not being used. The motivation for that change was that if a dependency happens to contain bitcode, ld64 will try to load libLTO without -flto explicitly being enabled. However, the change had the undesirable side effect of warning if libLTO.dylib doesn't exist even if it isn't needed.

Change things so that -lto_library is always passes, independent of if it exists or not. ld64 only looks at this flag if it uses LTO. If the dylib exists, all is well. If it doesn't, and LTO is not being used, all is well too. If ld64 does end up using LTO and the dylib does not exist, ld64 will print something like

ld: could not process llvm bitcode object file, because foo/libLTO.dylib could not be loaded file 'test.o' for architecture x86_64

[mehdi_amini]

LGTM, thanks!

[rnk]

lgtm, great idea!

[thakis]

287685, thanks!

include/clang/Basic/DiagnosticDriverKinds.td

def err_drv_expecting_fopenmp_with_fopenmp_targets : Error<
   "The option -fopenmp-targets must be used in conjunction with a -fopenmp option compatible with offloading, please use -fopenmp=libomp or -fopenmp=libiomp5.">;
 def warn_drv_omp_offload_target_duplicate : Warning<
   "The OpenMP offloading target '%0' is similar to target '%1' already specified - will be ignored.">, 
   InGroup<OpenMPTarget>;
 def err_drv_bitcode_unsupported_on_toolchain : Error<
   "-fembed-bitcode is not supported on versions of iOS prior to 6.0">;
 
 def warn_O4_is_O3 : Warning<"-O4 is equivalent to -O3">, InGroup<Deprecated>;
-def warn_drv_lto_libpath : Warning<"libLTO.dylib relative to clang installed dir not found; using 'ld' default search path instead">,
-  InGroup<LibLTO>;
 def warn_drv_optimization_value : Warning<"optimization level '%0' is not supported; using '%1%2' instead">,
   InGroup<InvalidCommandLineArgument>;
 def warn_ignored_gcc_optimization : Warning<"optimization flag '%0' is not supported">,
   InGroup<IgnoredOptimizationArgument>;
 def warn_drv_unsupported_opt_for_target : Warning<
   "optimization flag '%0' is not supported for target '%1'">,
   InGroup<IgnoredOptimizationArgument>;
 def warn_c_kext : Warning<

lib/Driver/Tools.cpp

if (Version[0] >= 116 && NeedsTempPath(Inputs)) {
           D.GetTemporaryPath("cc", types::getTypeTempSuffix(types::TY_Object)));
       C.addTempFile(TmpPath);
       CmdArgs.push_back("-object_path_lto");
       CmdArgs.push_back(TmpPath);
     }
   }
 
   // Use -lto_library option to specify the libLTO.dylib path. Try to find
-  // it in clang installed libraries. If not found, the option is not used
-  // and 'ld' will use its default mechanism to search for libLTO.dylib.
+  // it in clang installed libraries. ld64 will only look at this argument
+  // when it actually uses LTO, so libLTO.dylib only needs to exist at link
+  // time if ld64 decides that it needs to use LTO.
+  // Since this is passed unconditionally, ld64 will never look for libLTO.dylib
+  // next to it. That's ok since ld64 using a libLTO.dylib not matching the
+  // clang version won't work anyways.
   if (Version[0] >= 133) {
     // Search for libLTO in <InstalledDir>/../lib/libLTO.dylib
     StringRef P = llvm::sys::path::parent_path(D.Dir);
     SmallString<128> LibLTOPath(P);
     llvm::sys::path::append(LibLTOPath, "lib");
     llvm::sys::path::append(LibLTOPath, "libLTO.dylib");
-    if (llvm::sys::fs::exists(LibLTOPath)) {
-      CmdArgs.push_back("-lto_library");
-      CmdArgs.push_back(C.getArgs().MakeArgString(LibLTOPath));
-    } else {
-      D.Diag(diag::warn_drv_lto_libpath);
-    }
   }
 
   // ld64 version 262 and above run the deduplicate pass by default.
   if (Version[0] >= 262 && shouldLinkerNotDedup(C.getJobs().empty(), Args))
     CmdArgs.push_back("-no_deduplicate");
 
   // Derived from the "link" spec.
   Args.AddAllArgs(CmdArgs, options::OPT_static);

test/Driver/darwin-ld-lto.c

 // REQUIRES: system-darwin
 
-// Check that ld gets "-lto_library" and warnings about libLTO.dylib path.
+// Check that ld gets "-lto_library".
 
 // RUN: mkdir -p %T/bin
 // RUN: mkdir -p %T/lib
 // RUN: touch %T/lib/libLTO.dylib
 // RUN: %clang -target x86_64-apple-darwin10 -### %s \
 // RUN:   -ccc-install-dir %T/bin -mlinker-version=133 2> %t.log
 // RUN: FileCheck -check-prefix=LINK_LTOLIB_PATH %s -input-file %t.log
 //
 // LINK_LTOLIB_PATH: {{ld(.exe)?"}}
 // LINK_LTOLIB_PATH: "-lto_library"
 
+// Also pass -lto_library even if the file doesn't exist; if it's needed at
+// link time, ld will complain instead.
 // RUN: %clang -target x86_64-apple-darwin10 -### %s \
 // RUN:   -ccc-install-dir %S/dummytestdir -mlinker-version=133 2> %t.log
-// RUN: FileCheck -check-prefix=LINK_LTOLIB_PATH_WRN %s -input-file %t.log
-//
-// LINK_LTOLIB_PATH_WRN: warning: libLTO.dylib relative to clang installed dir not found; using 'ld' default search path instead
-
-// RUN: %clang -target x86_64-apple-darwin10 -### %s \
-// RUN:   -ccc-install-dir %S/dummytestdir -mlinker-version=133 -Wno-liblto 2> %t.log
-// RUN: FileCheck -check-prefix=LINK_LTOLIB_PATH_NOWRN %s -input-file %t.log
-//
-// LINK_LTOLIB_PATH_NOWRN-NOT: warning: libLTO.dylib relative to clang installed dir not found; using 'ld' default search path instead
+// RUN: FileCheck -check-prefix=LINK_LTOLIB_PATH %s -input-file %t.log