Use correct LLVM build dir for includes from unpackaged LLVM builds

`get_llvm_build_dir()` returns directory `<llvm_dir>/build`. It is incorrect when using non-packaged LLVM build, because LLVM is built into `'deps/llvm/'..get_llvm_package_name()`. This change fixes issue where using unpackaged LLVM build resulted in `error: ‘StrictFP’ is not a member of ‘llvm::Attribute’`.

build/LLVM.lua

@@ -25,7 +25,7 @@ function SearchLLVM()
 end
 
 function get_llvm_build_dir()
-  return path.join(LLVMRootDir, "build")
+  return path.join(LLVMRootDir, get_llvm_package_name())
 end
 
 function SetupLLVMIncludes()