Revert "Revert "[InstrProf] Support for external functions in text format.""

Summary:
This reverts commit 364eb09576a7667bc6d3ff80c52a83014ccac976 and separates out
the portion that was fixing binary reader error propagation - turns out, there
are production cases where that causes a regression.

Will re-introduce the error propagation fix separately.

The fix to the text reader error propagation is still "in".

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44807

llvm-svn: 328244