Speedup some unicode rendering

Use a fast path for column width computation for ascii characters. Especially
relevant for llvm-objdump.

before:

    % time ./bin/llvm-objdump -D  -j .text /lib/libc.so.6 >/dev/null
    ./bin/llvm-objdump -D -j .text /lib/libc.so.6 > /dev/null  0.75s user 0.01s system 99% cpu 0.757 total

after:

    % time ./bin/llvm-objdump -D  -j .text /lib/libc.so.6 >/dev/null
    ./bin/llvm-objdump -D -j .text /lib/libc.so.6 > /dev/null  0.37s user 0.01s system 99% cpu 0.378 total

Differential Revision: https://reviews.llvm.org/D92180