Add test for and close #12671, starting from non directory

use mktempdir instead of tempname because of #9053

Cygwin lets you do this but win32 doesn't

Only run test for 12671 if llvm is new enough

test/cmdlineargs.jl

@@ -239,4 +239,13 @@ let exename = `$(joinpath(JULIA_HOME, Base.julia_exename())) --precompiled=yes`
     @test readchomp(`$exename --compilecache=yes -E "Bool(Base.JLOptions().use_compilecache)"`) == "true"
     @test readchomp(`$exename --compilecache=no -E "Bool(Base.JLOptions().use_compilecache)"`) == "false"
     @test !success(`$exename --compilecache=foo -e "exit(0)"`)
+
+    # issue #12671, starting from a non-directory
+    @unix_only if VersionNumber(Base.libllvm_version) > v"3.3"
+        testdir = mktempdir()
+        cd(testdir) do
+            rm(testdir)
+            @test success(`$exename -e "exit(0)"`)
+        end
+    end
 end