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
