[gn] Use "$link /lib" for archives instead of lib.exe

[rnk]

Summary

This avoids the need to talk about lib.exe or llvm-lib.exe and it does
the right thing with LLD.

[thakis]

Does that do the right thing with link.exe? Should we only do this if use_lld?

[rnk]

In D60155#1452209, @thakis wrote:

Does that do the right thing with link.exe? Should we only do this if use_lld?

Yep, it does the right thing. link -lib -? prints the options for lib.exe. I figured this was simpler than managing the path for a separate tool.

[thakis]

TIL! It's not documented on https://docs.microsoft.com/en-us/cpp/build/reference/linker-options?view=vs-2019 but that now lld-link /lib makes much more sense to me :D

Thanks for the patch!

llvm/trunk/utils/gn/build/toolchain/BUILD.gn

tool("cxx") {
     depsformat = "msvc"
     description = "CXX {{output}}"
     outputs = [
       "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
     ]
   }
 
   tool("alink") {
-    command = "lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
+    command = "$link /lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
     description = "LIB {{output}}"
     outputs = [
       "{{output_dir}}/{{target_output_name}}.lib",
     ]
     default_output_dir = "{{root_out_dir}}/lib"
   }
 
   tool("solink") {