[flang] Fix clang build on MacOS

clang doesn't consider `std::strlen` of a constant to be a constexpr,
so just replace with the actual length.

Original-commit: flang-compiler/f18@738deab465
Reviewed-on: https://github.com/flang-compiler/f18/pull/761

flang/lib/semantics/mod-file.cc

@@ -43,7 +43,7 @@ using namespace parser::literals;
 struct ModHeader {
   static constexpr const char *bom{"\xef\xbb\xbf"};
   static constexpr const char *magic{"!mod$ v1 sum:"};
-  static constexpr int magicLen{std::strlen(magic)};
+  static constexpr int magicLen{13};
   static constexpr int sumLen{16};
   static constexpr char terminator{'\n'};
   static constexpr int len{magicLen + 1 + sumLen};