[PGO] Add --text option for llvm-profdata show|merge commands

The new option is similar to the SampleProfile dump option.

- dump raw/indexed format into text profile format
- merge the profile and output into text profile format.

Note that Value Profiling data text format is not yet designed. 
That functionality will be added later.

Differential Revision: http://reviews.llvm.org/D14894

llvm-svn: 253913