[flang] Add FLANG_PARALLEL_COMPILE_JOBS option (#95672)

This works the same way as LLVM_PARALLEL_COMPILE_JOBS except that it is
specific to the flang source rather than for the whole project.

Configuring with -DFLANG_PARALLEL_COMPILE_JOBS=1 would mean that there
would only ever be one flang source being compiled at a time.

Some of the flang sources require large amounts of memory to compile, so
this option can be used to avoid OOM erros when compiling those files
while still allowing the rest of the project to compile using the
maximum number of jobs.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

flang/CMakeLists.txt

diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index cbe8f11..75db5b9 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -445,6 +445,18 @@
   endif()
 endif()
 
+# Set up job pools for flang.  Some of the flang sources take a lot of memory to
+# compile, so allow users to limit the number of parallel flang jobs.  This is
+# useful for building flang alongside several other projects since you can use
+# the maximum number of build jobs for the other projects while limiting the
+# number of flang compile jobs.
+#
+# We want this set to infinity by default
+set(FLANG_PARALLEL_COMPILE_JOBS 0 CACHE STRING
+  "The maximum number of concurrent compilation jobs (Ninja only)")
+
+set_property(GLOBAL APPEND PROPERTY JOB_POOLS flang_compile_job_pool=${FLANG_PARALLEL_COMPILE_JOBS})
+
 include(CMakeParseArguments)
 include(AddFlang)
 

flang/cmake/modules/AddFlang.cmake

diff --git a/flang/cmake/modules/AddFlang.cmake b/flang/cmake/modules/AddFlang.cmake
index 3a5119b..c4b9573 100644
--- a/flang/cmake/modules/AddFlang.cmake
+++ b/flang/cmake/modules/AddFlang.cmake
@@ -89,6 +89,7 @@
       set_property(GLOBAL APPEND PROPERTY FLANG_LIBS ${name})
     endif()
     set_property(GLOBAL APPEND PROPERTY FLANG_EXPORTS ${name})
+    set_property(TARGET obj.${name} PROPERTY JOB_POOL_COMPILE flang_compile_job_pool)
   else()
     # Add empty "phony" target
     add_custom_target(${name})