[clang] [clang-format] Fix path expansion inside git-clang-format.bat (PR #114078)

[Nagy-Egri Máté Ferenc via cfe-commits]

https://github.com/MathiasMagnus created https://github.com/llvm/llvm-project/pull/114078

The trampoline script used on Windows (due to the absence of shebang support) doesn't properly expand the path to the Python script, as it leaves out the drive letter.

Functionally equivalent reproducer in action
```
PS C:\Users\mate> gc (gcm git-clang-formatish.bat).Source
@ECHO OFF
echo "%~pn0" %*
PS C:\Users\mate> git-clang-formatish
"\Users\mate\git-clang-formatish"
```

Adding `d` to the variable modifiers [as per the docs](https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/for) the drive letter is added. Even in the magical cases when it works.

(I couldn't reproduce, but I suspect it's only tested from some bash/cygwin variant, where the path becomes `/c/Program Files/...`, but the drive letter is needed. Without it, I also observed cases when used via `git clang-format` (without the inital dash) it tries to infer the drive letter based on the current working directory. In that case it fails to find `D:\Program Files\LLVM\bin\clang-format.exe`, which naturally fails, because `Program Files` is on `C:`)

>From 9a6a029cc06596ad020963a3d453df757e194120 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?M=C3=A1t=C3=A9=20Ferenc=20Nagy-Egri?= <mate at rastergrid.com>
Date: Tue, 29 Oct 2024 16:57:57 +0100
Subject: [PATCH] [clang-format] Expand drive letter in trampoline

---
 clang/tools/clang-format/git-clang-format.bat | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/tools/clang-format/git-clang-format.bat b/clang/tools/clang-format/git-clang-format.bat
index 19c82d8a04132b..a40276e63c5848 100644
--- a/clang/tools/clang-format/git-clang-format.bat
+++ b/clang/tools/clang-format/git-clang-format.bat
@@ -1 +1 @@
-py -3 "%~pn0" %*
+py -3 "%~dpn0" %*