[flang] (NFC) Break up flang/runtime/reduction.cpp

The single source file reduction.cpp is a little large in
terms of both source lines and generated text bytes, so
split SUM, PRODUCT, FINDLOC, and MAXLOC/MAXVAL/MINLOC/MINVAL
off into their own C++ source files that share a set of
implementation function templates now in a common header.

Differential Revision: https://reviews.llvm.org/D101111

Author: klausler

flang/runtime/CMakeLists.txt

@@ -42,7 +42,9 @@ add_flang_library(FortranRuntime
   edit-input.cpp
   edit-output.cpp
   environment.cpp
+  extrema.cpp
   file.cpp
+  findloc.cpp
   format.cpp
   internal-unit.cpp
   iostat.cpp
@@ -55,8 +57,10 @@ add_flang_library(FortranRuntime
   numeric.cpp
   random.cpp
   reduction.cpp
+  product.cpp
   stat.cpp
   stop.cpp
+  sum.cpp
   terminator.cpp
   tools.cpp
   transformational.cpp