Fork channel

Create a new channel as a copy of main.

Rename channel

Rename main to:

Delete channel

Delete main? This cannot be undone.

dump-fpm.test
RUN: llvm-pdbutil bytes -fpm %p/Inputs/empty.pdb | FileCheck %s

CHECK:                             Free Page Map
CHECK-NEXT: ============================================================
CHECK-NEXT: Block 2 (
CHECK-NEXT:   2000: 380300FE FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF  |8...............................|
CHECK-NEXT:   2020: FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF  |................................|
CHECK:        2FE0: FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF  |................................|
CHECK-NEXT: )