[llvm] MC: Allocate initial fragment and define section symbol in changeSection (PR #150574)

[via llvm-commits]

dyung wrote:

@MaskRay this change seems to be causing a test failure on the AArch64 MacOS bot, can you take a look?

https://lab.llvm.org/buildbot/#/builders/190/builds/24187

https://github.com/llvm/llvm-project/pull/150574