InstrProf: Simplify the construction of BinaryCoverageReader

Creating BinaryCoverageReader is a strange and complicated dance where
the constructor sets error codes that member functions will later
read, and the object is in an invalid state if readHeader isn't
immediately called after construction.

Instead, make the constructor private and add a static create method
to do the construction properly. This also has the benefit of removing
readHeader completely and simplifying the interface of the object.

llvm-svn: 230676