Patch unable to find file

I am currently in directory - /usr/lib/llvm-9/build

The include/ directory is a symlink -

reikdas@reikdas-VirtualBox:/usr/lib/llvm-9/build$ ls -l
lrwxrwxrwx 1 root root 10 Jan 31 2020 include -> ../include

But since I will be patching this via a script, I cannot manually put in the actual path of the symlink.

I have a .diff file that looks like this -

diff --git a/include/llvm/Demangle/MicrosoftDemangleNodes.h b/include/llvm/Demangle/MicrosoftDemangleNodes.h
index da9d9d5bfdc..3d47471f0ef 100644
--- a/include/llvm/Demangle/MicrosoftDemangleNodes.h
+++ b/include/llvm/Demangle/MicrosoftDemangleNodes.h
@@ -16,6 +16,8 @@
 #include "llvm/Demangle/DemangleConfig.h"
 #include "llvm/Demangle/StringView.h"
 #include <array>
+#include <cstdint>
+#include <string>
 
 namespace llvm {
 namespace itanium_demangle {

I do - patch -p1 < nameofthepatch.diff

but, I get an error -

Can't find file to patch at input line 3
Perhaps you used the wrong -p or --strip option?
The text leading upto this was:
....
File to patch:

But the file does exist -

reikdas@reikdas-VirtualBox:/usr/lib/llvm-9/build$ ls include/llvm/Demangle/ | grep MicrosoftDemangleNodes.h
MicrosoftDemangleNodes.h

I get the same error with - patch -p1 --follow-symlinks < nameofthepatch.diff

(please note that I specified which directory I executed these commands from)

The diff was generated on a Linux machine, so there are no hidden carriage returns.

Also, this works -

patch -p1 include/llvm/Demangle/MicrosoftDemangleNodes.h nameofthepatch.diff

which is not tenable, since a single .diff file might contain the diff for multiple files.

Why do I get this error?