Properly support LLVM_ENABLE_LLD on Windows

[serge-sans-paille]

Summary

Currently, setting -DLLVM_ENABLE_LLD=ON on windows also requires setting
-DCMAKE_LINKER=lld-link.exe. This is both misleading and redundant.

Fix this by trying to find llvm-link.exe when -DLLVM_ENABLE_LLD=ON is
set and CMAKE_LINKER is not, and aborting otherwise.

[phosek]

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

307

Nit: I'd use a slightly different phrasing.

llvm/cmake/modules/HandleLLVMOptions.cmake

if (${idx} GREATER -1)
     endif()
   endif()
 endfunction()
 
 if( LLVM_ENABLE_LLD )
   if ( LLVM_USE_LINKER )
     message(FATAL_ERROR "LLVM_ENABLE_LLD and LLVM_USE_LINKER can't be set at the same time")
   endif()
+
   # In case of MSVC cmake always invokes the linker directly, so the linker
   # should be specified by CMAKE_LINKER cmake variable instead of by -fuse-ld
   # compiler option.
-  if ( NOT MSVC )
+  if ( MSVC )
+    if(NOT CMAKE_LINKER MATCHES "lld-link")
+        get_filename_component(CXX_COMPILER_DIR ${CMAKE_CXX_COMPILER} DIRECTORY)
+        get_filename_component(C_COMPILER_DIR ${CMAKE_C_COMPILER} DIRECTORY)
+        find_program(LLD_LINK NAMES "lld-link" "lld-link.exe" HINTS ${CXX_COMPILER_DIR} ${C_COMPILER_DIR} DOC "lld linker")
+        if(NOT LLD_LINK)
+            message(FATAL_ERROR
+                "LLVM_ENABLE_LLD set, but cannot find lld-link. "
+                "Consider setting CMAKE_LINKER to lld-link path.")
+        endif()
+        set(CMAKE_LINKER ${LLD_LINK})
+    endif()
+  else()
     set(LLVM_USE_LINKER "lld")
   endif()
 endif()
 
 if( LLVM_USE_LINKER )
   append("-fuse-ld=${LLVM_USE_LINKER}"
     CMAKE_EXE_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   check_cxx_source_compiles("int main() { return 0; }" CXX_SUPPORTS_CUSTOM_LINKER)