[flang] Implement reductions in the runtime

[flang] Implement reductions in the runtime

Add runtime APIs, implementations, and tests for ALL, ANY, COUNT,
MAXLOC, MAXVAL, MINLOC, MINVAL, PRODUCT, and SUM reduction
transformantional intrinsic functions for all relevant argument
and result types and kinds, both without DIM= arguments
(total reductions) and with (partial reductions).

Complex-valued reductions have their APIs in C so that
C's _Complex types can be used for their results.

Some infrastructure work was also necessary or noticed:
* Usage of "long double" in the compiler was cleaned up a
  bit, and host dependences on x86 / MSVC have been isolated
  in a new Common/long-double header.
* Character comparison has been exposed via an extern template
  so that reductions could use it.
* Mappings from Fortran type category/kind to host C++ types
  and vice versa have been isolated into runtime/cpp-type.h and
  then used throughout the runtime as appropriate.
* The portable 128-bit integer package in Common/uint128.h
  was generalized to support signed comparisons.
* Bugs in descriptor indexing code were fixed.

Differential Revision: https://reviews.llvm.org/D99666