[libclc] libclc: clspv: update gen_convert.cl for clspv (PR #66902)

[Kévin Petit via cfe-commits]

================
@@ -26,6 +27,12 @@
 #
 # convert_<destTypen><_sat><_roundingMode>(<sourceTypen>)
 
+import sys
+
+clspv = False
+if len(sys.argv) == 2 and sys.argv[1] == "--clspv":
+    clspv = True
+
----------------
kpet wrote:

```suggestion
import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--clspv', help="Generate the clspv variant of the code")
args = parser.parse_args()
clspv = args.clspv
```
It's not that much more code and will be easier to maintain, use, and extend. 

https://github.com/llvm/llvm-project/pull/66902