[Flang] Fix path strings in CreateFIRLangRef.py

In D129186, CreateFIRLangRef.py was created to help fix
formatting problems with FIRLangRef.md.

However, due to an error while rebasing, the last edit was lost,
as such the current path variables are Unix-style.

This patch addresses what was discussed in the previous patch and
now uses os.path.join instead.

Differential Revision: https://reviews.llvm.org/D129644

flang/docs/FIR/CreateFIRLangRef.py

@@ -1,17 +1,19 @@
 # This script combines FIRLangRef_Header.md with the auto-generated Dialect/FIRLangRef.md
-# for the purpose of creating an introduction header/paragraph for FIRLangRef.html
+# for the purpose of creating an introduction header/paragraph for FIRLangRef.html.
 
-# These paths are relative from the build directroy, not source, as that's where this tool is exectued.
+import os
-header_path = 'Source/FIR/FIRLangRef_Header.md'
+
-docs_path   = 'Dialect/FIRLangRef.md'
+# These paths are relative to flang/docs in the build directory, not source, as that's where this tool is executed.
-output_path = 'Source/FIRLangRef.md'
+HEADER_PATH = os.path.join('Source', 'FIR', 'FIRLangRef_Header.md')
+DOCS_PATH   = os.path.join('Dialect', 'FIRLangRef.md')
+OUTPUT_PATH = os.path.join('Source', 'FIRLangRef.md')
 
 # 1. Writes line 1 from docs to output, (comment line that the file is autogenerated)
 # 2. Adds a new line
 # 3. Writes the entire header to the output file
 # 4. Writes the remainder of docs to the output file
-with open(output_path, 'w') as output:
+with open(OUTPUT_PATH, 'w') as output:
-    with open(header_path, 'r') as header, open(docs_path, 'r') as docs:
+    with open(HEADER_PATH, 'r') as header, open(DOCS_PATH, 'r') as docs:
         output.write(docs.readline())
         output.write("\n")
         output.write(header.read())