[llvm-profdata] Rename show flag to --show-format

In https://reviews.llvm.org/D135127 we created the show flag
`--output-format` which was confusing because it behaved differently
than the same flag in the merge command. So, rename the flag to
`--show-format`. This also allows us to add the `text` option to mean
"normal text output" rather than "text-encoded profiles" like it does
for the merge command.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D135467