[clang] [clang] Only build clang-tblgen if it is actually needed (PR #161952)

[Ross Burton via cfe-commits]

https://github.com/rossburton updated https://github.com/llvm/llvm-project/pull/161952

>From 98669ac9410e5b8c46c104f2ed2a4f91c4268250 Mon Sep 17 00:00:00 2001
From: Ross Burton <ross.burton at arm.com>
Date: Fri, 3 Oct 2025 20:34:02 +0100
Subject: [PATCH] [clang] Only build clang-tblgen if it is actually needed

It's possible to build clang with an existing clang-tblgen (common when
cross-compiling, for instance) by setting CLANG_TABLEGEN_EXE.  If this
is the case then building another copy of clang-tblgen that won't be
used is pointless.

However, currently the clang tests explicitly depend on clang-tblgen
being built, so also check if CLANG_INCLUDE_TESTS is enabled to ensure
that the targets exist.
---
 clang/CMakeLists.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index e4cb1a359620d..b650b3b986f4c 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -479,7 +479,9 @@ option(CLANG_ENABLE_HLSL "Include HLSL build products" Off)
 # While HLSL support is experimental this should stay hidden.
 mark_as_advanced(CLANG_ENABLE_HLSL)
 
-add_subdirectory(utils/TableGen)
+if (NOT DEFINED CLANG_TABLEGEN_EXE OR CLANG_INCLUDE_TESTS)
+  add_subdirectory(utils/TableGen)
+endif()
 
 # Export CLANG_TABLEGEN_EXE for use by flang docs.
 set(CLANG_TABLEGEN_EXE "${CLANG_TABLEGEN_EXE}" CACHE INTERNAL "")