[cmake] Enable thin lto cache when building with lld-link

[russell.gallop]

Summary

This was enabled for other platforms. Added option for Windows/lld-link.

[rnk]

@inglorion ended up disabling the thinlto cache on Windows. I've been assuming it is buggy on Windows, i.e. it relies on filesystem invariants not provided there. But, if it works for you, I'm happy to try it. We continuously build some tools such as LLD in this configuration in Chromium, so I think we'll just pick this up and see if it works out of the box.

lgtm

[russell.gallop]

Thanks.

Maybe this is what you're referring to: https://bugs.llvm.org/show_bug.cgi?id=38616

It seems to work for me, though I may not have done as much testing as @inglorion.

We document this option (https://clang.llvm.org/docs/ThinLTO.html#incremental) so I'd like to try enabling it.

llvm/cmake/modules/HandleLLVMOptions.cmake

if(APPLE)
     append("-Wl,-cache_path_lto,${PROJECT_BINARY_DIR}/lto.cache"
            CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   elseif(UNIX AND LLVM_USE_LINKER STREQUAL "lld")
     append("-Wl,--thinlto-cache-dir=${PROJECT_BINARY_DIR}/lto.cache"
            CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   elseif(LLVM_USE_LINKER STREQUAL "gold")
     append("-Wl,--plugin-opt,cache-dir=${PROJECT_BINARY_DIR}/lto.cache"
            CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  elseif(LINKER_IS_LLD_LINK)
+    append("/lldltocache:${PROJECT_BINARY_DIR}/lto.cache"
+           CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
 elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
   append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)
     append("-flto=full" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
 elseif(LLVM_ENABLE_LTO)
   append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)