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

[Ivan Kosarev via llvm-commits]

kosarev wrote:

@jayfoad Would you mind to let https://github.com/llvm/llvm-project/pull/160316 go first, so this doesn't interfere?

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