[llvm-profdata] Improve error messaging when merging mismatched profile data

Summary:
This change tries to make the root cause of instrumented profile data merge failures clearer.

Previous:

$ llvm-profdata merge test_0.profraw test_1.profraw -o test_merged.profdata
test_1.profraw: foo: Function count mismatch
test_1.profraw: bar: Function count mismatch
test_1.profraw: baz: Function count mismatch
...

Changed:

$ llvm-profdata merge test_0.profraw test_1.profraw -o test_merged.profdata
test_1.profraw: foo: Function basic block count change detected (counter mismatch)
Make sure that all profile data to be merged is generated from the same binary.
test_1.profraw: bar: Function basic block count change detected (counter mismatch)
test_1.profraw: baz: Function basic block count change detected (counter mismatch)
...

Reviewers: dnovillo, davidxl, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14739

llvm-svn: 253384