[llvm-profdata] Clean up the way we create the input filenames buffer (NFC)

[llvm-profdata] Clean up the way we create the input filenames buffer (NFC)

Create the buffer before calling parseInputFilenamesFile(), and add a
comment explaining why this is done.

Thanks to David Li for the suggestion!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271756 91177308-0d34-0410-b5e6-96231b3b80d8