test: Use llvm-profdata merge in Profile tests

In preparation for using a binary format for instrumentation based
profiling, explicitly treat the test inputs as text and transform them
before running. This will allow us to leave the checked in files in
human readable format once the instrumentation format is binary.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206509 91177308-0d34-0410-b5e6-96231b3b80d8