Re-apply r249644: Handle inline stacks in gcov-encoded sample profiles.

Re-apply r249644: Handle inline stacks in gcov-encoded sample profiles.

This fixes memory allocation problems by making the merge operation keep
the profile readers around until the merged profile has been emitted.
This is needed to prevent the inlined function names to disappear from
the function profiles. Since all the names are kept as references, once
the reader disappears, the names are also deallocated.

Additionally, XFAIL on big-endian architectures. The test case uses a
gcov file generated on a little-endian system.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249724 91177308-0d34-0410-b5e6-96231b3b80d8