[llvm] Improve error message for cmake failure (PR #71404)

[Jeremy Kun via llvm-commits]

https://github.com/j2kun updated https://github.com/llvm/llvm-project/pull/71404

>From fb47f68f3bd5ce7110a29a979bf73c7bebd3faf5 Mon Sep 17 00:00:00 2001
From: Jeremy Kun <jkun at google.com>
Date: Mon, 6 Nov 2023 07:04:30 -0800
Subject: [PATCH] improve error message for cmake failure

---
 llvm/cmake/modules/LLVMProcessSources.cmake | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/llvm/cmake/modules/LLVMProcessSources.cmake b/llvm/cmake/modules/LLVMProcessSources.cmake
index 340aa7b43eaed96..205089bd2704cb5 100644
--- a/llvm/cmake/modules/LLVMProcessSources.cmake
+++ b/llvm/cmake/modules/LLVMProcessSources.cmake
@@ -113,7 +113,12 @@ function(llvm_check_source_file_list)
           else()
               set(fn_relative "${fn}")
           endif()
-          message(SEND_ERROR "Found unknown source file ${fn_relative}
+          message(SEND_ERROR "Found erroneous configuration for source file ${fn_relative}
+LLVM's build system enforces that all source files are added to a build target, \
+that exactly one build target exists in each directory, \
+and that this target lists all files in that directory. \
+If you want multiple targets in the same directory, add \
+PARTIAL_SOURCES_INTENDED to the target specification, though it is discouraged.
 Please update ${CMAKE_CURRENT_LIST_FILE}\n")
         endif()
       endif()