[flang] Change uniqueCGIdent separator from . to X (llvm#71338)

TODO: Include additional changes to unit tests

Author: georgestagg

flang/lib/Optimizer/Builder/FIRBuilder.cpp

@@ -959,13 +959,13 @@ std::string fir::factory::uniqueCGIdent(llvm::StringRef prefix,
     llvm::SmallString<32> str;
     llvm::MD5::stringifyResult(result, str);
     std::string hashName = prefix.str();
-    hashName.append(".").append(str.c_str());
+    hashName.append("X").append(str.c_str());
     return fir::NameUniquer::doGenerated(hashName);
   }
   // "Short" identifiers use a reversible hex string
   std::string nm = prefix.str();
   return fir::NameUniquer::doGenerated(
-      nm.append(".").append(llvm::toHex(name)));
+      nm.append("X").append(llvm::toHex(name)));
 }
 
 mlir::Value fir::factory::locationToFilename(fir::FirOpBuilder &builder,