[flang][driver] Add -l to the list of "visble" flags

Missed in https://reviews.llvm.org/D157837.

Fixes:
  * https://lab.llvm.org/buildbot/#/builders/197/builds/8996/steps/13/logs/stdio

clang/include/clang/Driver/Options.td

@@ -4132,7 +4132,7 @@ def imultilib : Separate<["-"], "imultilib">, Group<gfortran_Group>;
 def K : Flag<["-"], "K">, Flags<[LinkerInput]>;
 def keep__private__externs : Flag<["-"], "keep_private_externs">;
 def l : JoinedOrSeparate<["-"], "l">, Flags<[LinkerInput, RenderJoined]>,
-        Group<Link_Group>;
+        Visibility<[ClangOption, FlangOption]>, Group<Link_Group>;
 def lazy__framework : Separate<["-"], "lazy_framework">, Flags<[LinkerInput]>;
 def lazy__library : Separate<["-"], "lazy_library">, Flags<[LinkerInput]>;
 def mlittle_endian : Flag<["-"], "mlittle-endian">, Group<m_Group>,

flang/test/Driver/misc-flags.f90

@@ -0,0 +1,6 @@
+! Make sure that `-l` is "visible" to Flang's driver
+! RUN: %flang -lpgmath -### %s
+
+program hello
+  write(*,*), "Hello world!"
+end program hello