Update .gitmodules to reflect LLVM bump to 18.1

[JohRust]

Fixes #1611 The LLVM submodule was updated to 18.1 in commit 97fce6f and now points to https://github.com/llvm/llvm-project/tree/3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff. Make .gitmodules point to that branch as well.

[TommyMurphyTM1234]

LGTM - thanks. 👍

Just for completeness/reference purposes the previous PR bumping LLVM but forgetting to update .gitmodules is here:

• https://github.com/riscv-collab/riscv-gnu-toolchain/pull/1523