Currently these functions report errors directly to stderr, this updates
them to use diagnostics instead. This also makes partially-consumed
strings an error if the `numRead` parameter isn't provided (the
docstrings already claimed this happened, but it didn't.)
While here I also tried to reduce the number of overloads by switching
to using default parameters.
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D144804
f5f8a46bb0