ProfileData: Allow multiple profiles in RawInstrProfReader

Allow multiple raw profiles to coexist in a single .profraw file,
given the following conditions:

- Zero padding at the end of or between profiles will be skipped.
- Each profile must start with a valid header.
- Mixing endianness or pointer sizes in concatenated profiles files is
  not allowed.

This is needed to handle cases where a program's shared libraries are
profiled as well as the main executable itself, as we'll need to emit
each executable's counters. Combining the tables in the runtime would
be expensive for the instrumented program.

rdar://16918688