use re.I instead of [Xx] for each letter

Co-authored-by: Michael Kruse <[email protected]>

Author: kparzysz

flang/test/Semantics/test_symbols.py

@@ -29,7 +29,7 @@
 # Strips out blank lines and all comments except for "!DEF:", "!REF:", "!$acc" and "!$omp"
 with open(src, "r") as text_in:
     for line in text_in:
-        text = re.sub(r"!(?![DR]EF:|\$[Oo][Mm][Pp]|\$[Aa][Cc][Cc]).*", "", line)
+        text = re.sub(r"!(?![DR]EF:|\$omp|\$acc).*", "", line, re.I)
         text = re.sub(r"^\s*$", "", text)
         diff1 += text