[llvm] [MC] Add and use const overload of MCAsmParser::getContext. NFC. (PR #160490)

[Jay Foad via llvm-commits]

jayfoad wrote:

I will rethink this in the light of https://github.com/llvm/llvm-project/pull/160316/files#r2375811664 and come up with a new patch.

https://github.com/llvm/llvm-project/pull/160490