Fix issues with using a non-default system LLVM with CHPL_LLVM_CONFIG

util/chplenv/chpl_llvm.py

@@ -1024,6 +1024,42 @@ def get_clang_prgenv_args():
 
     return (comp_args, link_args)
 
+@memoize
+def get_system_include_directories(flag, lang):
+    """
+    Runs 'COMPILER -E -Wp,-v -' to determine the system include directories
+    """
+    directories = []
+    compiler = chpl_compiler.get_compiler_command(flag, lang)
+    _, _, stdout, _ = try_run_command(
+        compiler + ["-E", "-Wp,-v", "-", "-o", "/dev/null"],
+        combine_output=True, cmd_input=""
+    )
+    if stdout:
+        lines = stdout.splitlines()
+        start_includes_regex = re.compile(r"#include.+search starts here")
+        end_includes_regex = re.compile(r"End of search list")
+        ignoring_regex = re.compile(r"(?:ignoring nonexistent)|(?:as it is a non-system)")
+        collecting = False
+        for line in lines:
+            if start_includes_regex.search(line):
+                collecting = True
+            elif end_includes_regex.search(line):
+                collecting = False
+                break
+            elif collecting and not ignoring_regex.search(line):
+                directories.append(line.strip())
+    return directories
+
+@memoize
+def is_path_system_include_directory(path, flag, lang):
+    """
+    Returns True if the given path is a system include directory
+    """
+    directories = get_system_include_directories(flag, lang)
+    p = os.path.realpath(os.path.abspath(path))
+    return any([os.path.realpath(os.path.abspath(d)) == p for d in directories])
+
 # Filters out C++ compilation flags from llvm-config.
 # The flags are passed as a list of strings.
 # Returns a list of strings containing the kept flags.
@@ -1048,18 +1084,25 @@ def filter_llvm_config_flags(llvm_val, flags):
             continue # filter out these flags
 
         #
-        # include LLVM headers as system headers
+        # include LLVM headers as system headers using -isystem
         # this avoids warnings inside of LLVM headers by treating LLVM headers
         #
-        # when adding LLVM=system as system headers, we should not perturb the
-        # include search path, so use -isystem-after/-idirafter
+        # If the header is already a system header, using -isystem will break
+        # the include search. In that case, just use -I (which technically wont do anythings)
         #
-        # when adding LLVM=bundled, we should include the LLVM headers as system
-        # headers and prefer the bundled headers, so use -isystem
-        #
-        include_flag = '-idirafter' if llvm_val == 'system' else '-isystem'
         if flag.startswith('-I'):
-            ret.append(include_flag + flag[2:])
+            directory = flag[2:]
+            if llvm_val == "system":
+                if not is_path_system_include_directory(directory, 'host', 'c++'):
+                    # its not included by the compiler, so its safe to use -isystem
+                    ret.append('-isystem' + directory)
+                else:
+                    # technically don't need to add this for
+                    # but it's harmless to use -I
+                    ret.append("-I" + directory)
+            else:
+                # bundled LLVM is always safe to use -isystem
+                ret.append("-isystem" + directory)
             continue
 
         if flag.startswith('-W'):