[llvm-objdump][NFC] Make the PrettyPrinter::printInst() output buffered

[llvm-objdump][NFC] Make the PrettyPrinter::printInst() output buffered

Summary:
Every time PrettyPrinter::printInst is called, stdout is flushed and it makes llvm-objdump slow. This patches adds a string
buffer to prevent stdout from being flushed.

Benchmark results (./llvm-objdump-master: without this patch,  ./bin/llvm-objcopy: with this patch):

  $ hyperfine --warmup 10 './llvm-objdump-master -d ./bin/llvm-objcopy' './bin/llvm-objdump -d ./bin/llvm-objcopy'
  Benchmark #1: ./llvm-objdump-master -d ./bin/llvm-objcopy
    Time (mean ± σ):      2.230 s ±  0.050 s    [User: 1.533 s, System: 0.682 s]
    Range (min … max):    2.115 s …  2.278 s    10 runs

  Benchmark #2: ./bin/llvm-objdump -d ./bin/llvm-objcopy
    Time (mean ± σ):     386.4 ms ±  13.0 ms    [User: 376.6 ms, System: 6.1 ms]
    Range (min … max):   366.1 ms … 407.0 ms    10 runs

  Summary
    './bin/llvm-objdump -d ./bin/llvm-objcopy' ran
      5.77 ± 0.23 times faster than './llvm-objdump-master -d ./bin/llvm-objcopy'

Reviewers: alexshap, Bigcheese, jhenderson, rupprecht, grimar, MaskRay

Reviewed By: jhenderson, MaskRay

Subscribers: dexonsmith, jhenderson, javed.absar, kristof.beyls, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64969

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366984 91177308-0d34-0410-b5e6-96231b3b80d8