[clang] [Matrix] Preserve signedness when extending matrix index expression. (PR #103044)

[John McCall via cfe-commits]

rjmccall wrote:

Do you just want whatever the logic is for array subscripts?  I believe array subscripts technically leave the index unconverted, and then we perform the necessary conversion when emitting IR.

https://github.com/llvm/llvm-project/pull/103044