[InstrProf] Support for external functions in text format.

Summary:
External functions appearing as indirect call targets could not be
found in the SymTab, and the value:counter record was represented,
in the text format, using an empty string for the name. This would
then cause a silent parsing error when reading.

This CL:
- adds explicit support for such functions
- fixes the places where we would not propagate errors when reading
- addresses a performance issue due to eager resorting of the SymTab.

Reviewers: xur, eraman, davidxl

Reviewed By: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44717

llvm-svn: 328132