[build] Fix --skip-build-llvm so that its minimal targets, like tblgen, are still built

Backported from commit c969814, resolves SR-13267

Author: finagolfin

utils/build-script-impl

@@ -1141,10 +1141,14 @@ LIBDISPATCH_STATIC_SOURCE_DIR="${WORKSPACE}/swift-corelibs-libdispatch"
 LIBICU_SOURCE_DIR="${WORKSPACE}/icu"
 LIBCXX_SOURCE_DIR="${WORKSPACE}/llvm-project/libcxx"
 
+# We cannot currently apply the normal rules of skipping here for LLVM. Even if
+# we are skipping building LLVM, we still need to at least build a few tools
+# like tblgen that Swift relies on for building and testing. See the LLVM
+# configure rules.
+PRODUCTS=(llvm)
 [[ "${SKIP_BUILD_CMARK}" ]] || PRODUCTS+=(cmark)
 [[ "${SKIP_BUILD_LIBCXX}" ]] || PRODUCTS+=(libcxx)
 [[ "${SKIP_BUILD_LIBICU}" ]] || PRODUCTS+=(libicu)
-[[ "${SKIP_BUILD_LLVM}" ]] || PRODUCTS+=(llvm)
 [[ "${SKIP_BUILD_SWIFT}" ]] || PRODUCTS+=(swift)
 [[ "${SKIP_BUILD_LLDB}" ]] || PRODUCTS+=(lldb)
 [[ "${SKIP_BUILD_LIBDISPATCH}" ]] || PRODUCTS+=(libdispatch)
@@ -1534,7 +1538,7 @@ for host in "${ALL_HOSTS[@]}"; do
                 if [ "${BUILD_LLVM}" == "0" ] ; then
                     build_targets=(clean)
                 fi
-                if [ "${SKIP_BUILD}" ] ; then
+                if [[ "${SKIP_BUILD}" || "${SKIP_BUILD_LLVM}" ]] ; then
                     # We can't skip the build completely because the standalone
                     # build of Swift depend on these for building and testing.
                     build_targets=(llvm-tblgen clang-resource-headers intrinsics_gen clang-tablegen-targets)

validation-test/BuildSystem/skip_cmark_swift_llvm.test-sh

@@ -15,7 +15,7 @@
 # SKIP-CMARK-CHECK: cmake --build {{.*}}swift-
 
 # SKIP-LLVM-CHECK: cmake --build {{.*}}cmark-
-# SKIP-LLVM-CHECK-NOT: cmake --build {{.*}}llvm-
+# SKIP-LLVM-CHECK: cmake --build {{.*}}llvm-tblgen
 # SKIP-LLVM-CHECK: cmake --build {{.*}}swift-
 
 # SKIP-SWIFT-CHECK: cmake --build {{.*}}cmark-