Changes

[demangle] Copy back some NFC commits from LLVM

[PowerPC] The VK_PLT symbolref modifier is only used on 32-bit ELF.
[NFC]
Fix a conditional that guarded code for execution only on 32-bit ELF by 
checking that the Subtarget was not 64-bit and not-Darwin. By adding a
new target ABI (AIX), the condition is no longer correct. This code is
dead for AIX, due to a 'report_fatal_error' for thread local storage
usage earlier in the pipeline, but needs to be modifed as part of
Darwins removal from the PowerPC backend.

[X86] AMD Znver2 (Rome) Scheduler enablement
The patch gives out the details of the znver2 scheduler model. There are
few improvements with respect to execution units, latencies and 
throughput when compared with znver1. The tests that were present for
znver1 for llvm-mca tool were replicated. The latencies, execution
units, timeline and throughput information are updated for znver2.
Reviewers: craig.topper, Simon Pilgrim
Differential Revision: https://reviews.llvm.org/D66088