[flang] Allow more concurrently open NEWUNIT= values, with recycling

Add a header-only implementation of Briggs & Torczon's fast small
integer set data structure to flang/include/flang/Common, and use
it in the runtime to manage a pool of Fortran unit numbers with
recycling.  This replaces the bit set previously used for that
purpose.  The set is initialized on demand with the negations of
all the NEWUNIT= unit numbers that can be returned to any kind
of integer variable.

For programs that require more concurrently open NEWUNIT= unit
numbers than the pool can hold, they are now allocated with a
non-recycling counter.  This allows as many open units as the
operating system provides.

Many of the top-line comments in flang/unittests/Runtime had the
wrong path name.  I noticed this while adding a unit test for the
fast integer set data structure, and cleaned them up.

Differential Revision: https://reviews.llvm.org/D120685