[llvm-mca] Add support for MCWriteProcResEntry::StartAtCycle

[RKSimon]

The ability to control resource intervals is currently being discussed:

https://reviews.llvm.org/D150310
https://reviews.llvm.org/D150311
https://reviews.llvm.org/D150312

We will need to add support to llvm-mca, along with some cases from the x86 scheduler models so we can best determine how to efficiently add support without too much bloat/refactoring.

CC @boomanaiden154

[llvmbot]

@llvm/issue-subscribers-tools-llvm-mca