[openmp] [test] Fix prepending config.library_dir to LD_LIBRARY_PATH

[mgorny]

Summary

Fix the LD_LIBRARY_PATH prepending order to make sure that
config.library_path ends up before any potentially-system directories
(e.g. config.hwloc_library_dir). This makes sure that we are testing
against the just-built openmp libraries rather than the version that is
already installed.

Also rename the function to prepend_* to make it clearer what it
actually does.

Bug #56821

[mgorny]

@jlpeyton, if this looks good, could you also approve backporting to 15.x in https://github.com/llvm/llvm-project/issues/56821 ?

[jlpeyton]

LGTM

openmp/runtime/test/lit.cfg

 # -*- Python -*- vim: set ft=python ts=4 sw=4 expandtab tw=79:
 # Configuration file for the 'lit' test runner.
 
 import os
 import re
 import subprocess
 import lit.formats
 
 # Tell pylint that we know config and lit_config exist somewhere.
 if 'PYLINT_IMPORT' in os.environ:
     config = object()
     lit_config = object()
 
-def append_dynamic_library_path(path):
+def prepend_dynamic_library_path(path):
     if config.operating_system == 'Windows':
         name = 'PATH'
         sep = ';'
     elif config.operating_system == 'Darwin':
         name = 'DYLD_LIBRARY_PATH'
         sep = ':'
     else:
         name = 'LD_LIBRARY_PATH'
if config.has_libatomic:
     libs += " -latomic"
 
 # Allow REQUIRES / UNSUPPORTED / XFAIL to work
 config.target_triple = [ ]
 for feature in config.test_compiler_features:
     config.available_features.add(feature)
 
 # Setup environment to find dynamic library at runtime
-append_dynamic_library_path(config.library_dir)
 if config.using_hwloc:
-    append_dynamic_library_path(config.hwloc_library_dir)
+    prepend_dynamic_library_path(config.hwloc_library_dir)
     config.available_features.add('hwloc')
+# Note: please keep config.library_dir *after* any potentially system
+# directories, as otherwise preinstalled openmp libraries will be used
+# over just-built
+prepend_dynamic_library_path(config.library_dir)
 
 # Rpath modifications for Darwin
 if config.operating_system == 'Darwin':
     config.test_flags += " -Wl,-rpath," + config.library_dir
     if config.using_hwloc:
         config.test_flags += " -Wl,-rpath," + config.hwloc_library_dir
 
 # Find the SDK on Darwin