[LLVM][AArch64] Explain that X19 is used as the frame base pointer register

Fixes #50098

LLVM uses X19 as the frame base pointer, if it needs to. Meaning you
can get warnings if you clobber that with inline asm.

However, it doesn't explain why. The frame base register is not part
of the ABI so it's pretty confusing why you get that warning out of the blue.

This adds a method to explain a reserved register with X19 as the first one.
The logic is the same as getReservedRegs.

I could have added a return parameter to isASMClobberable and friends
but found that there's a lot of things that call isReservedReg in various
ways.

So while one more method on the pile isn't great design, it is simpler
right now to do it this way and only pay the cost if you are actually using
a reserved register.

Reviewed By: lenary

Differential Revision: https://reviews.llvm.org/D133213