Reland "[gn build] (manually) port 6316129"

This reverts commit 21b97df.
6316129 relanded in 7b98391.

Author: nico

llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn

@@ -349,6 +349,7 @@ write_cmake_config("llvm-config") {
     "LLVM_NATIVE_TARGETINFO=1",
     "LLVM_NATIVE_TARGETMC=1",
     "LLVM_NATIVE_TARGETMCA=1",
+    "LLVM_UNREACHABLE_OPTIMIZE=1",
     "LLVM_USE_INTEL_JITEVENTS=",
     "LLVM_USE_OPROFILE=",
     "LLVM_USE_PERF=",