[clangd] Allow to build Clangd without decision forest

Make it possible to disable building the decision forest ranking
model for clangd.  To unbreak build of Clangd on PPC32 in gentoo, see
https://bugs.gentoo.org/829602

Based on D138520.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D139107
diff --git a/clang-tools-extra/clangd/Feature.cpp b/clang-tools-extra/clangd/Feature.cpp
index 5664a88..8ff7294 100644
--- a/clang-tools-extra/clangd/Feature.cpp
+++ b/clang-tools-extra/clangd/Feature.cpp
@@ -66,6 +66,10 @@
 #if !CLANGD_TIDY_CHECKS
       "-tidy"
 #endif
+
+#if !CLANGD_DECISION_FOREST
+      "-decision_forest"
+#endif
       ;
 }