[flang] Implement READ(SIZE=) and INQUIRE(IOLENGTH=) in runtime

Count input characters corresponding to formatted edit descriptors
for READ(SIZE=); count output bytes for INQUIRE(IOLENGTH=).

The I/O APIs GetSize() and GetLength() were adjusted to return
std::size_t as function results.

Basic unit tests were added (and others fixed).

Differential Revision: https://reviews.llvm.org/D110291