[llvm-rc] Support joined or separate spelling for /fo flag

[phosek]

Summary

CMake invokes rc using the joined spelling which appears to be supported
by Microsoft's rc implementation, so we should support it as well.

[mstorsjo]

Could you add a test for this? (I guess it's easiest to change one of the existing tests to use a different syntax in one place.)

Also, this isn't the only option that can be JoinedOrSeparate, with a quick test it seems like this goes for at least CODEPAGE and DEFINE as well, probably for all of them. But I won't demand fixing all of them at once, as they're tangential to what you need, and many of them aren't really implemented in llvm-rc yet.

[phosek]

Test added, I'd be fine adding tests for more flags in subsequent changes.

[mstorsjo]

LGTM

Fwiw, if you use llvm-rc with flags starting with a dash instead of a slash, llvm-rc -FO/path/to/out.res input.rc does run successfully and silently (but doesn't output the file in the intended place, the option just appears to be ignored). That might be an issue someone (myself included) perhaps should take a look at at some point...

llvm/test/tools/llvm-rc/flags.test

+; RUN: llvm-rc /dry-run /FO %t %p/Inputs/empty.rc 2>&1 | FileCheck %s --allow-empty --check-prefix=FO
+; RUN: llvm-rc /dry-run /FO%t %p/Inputs/empty.rc 2>&1 | FileCheck %s --allow-empty --check-prefix=FO
+
+; FO-NOT: Exactly one input file should be provided.

llvm/tools/llvm-rc/Opts.td

 include "llvm/Option/OptParser.td"
 
 // All the switches can be preceded by either '/' or '-'.
 // These options seem to be important for the tool
 // and should be implemented.
 
-def FILEOUT : Separate<[ "/", "-" ], "FO">,
+def FILEOUT : JoinedOrSeparate<[ "/", "-" ], "FO">,
               HelpText<"Change the output file location.">;
 
 def DEFINE : Separate<[ "/", "-" ], "D">,
              HelpText<"Define a symbol for the C preprocessor.">;
 def UNDEF : Separate<[ "/", "-" ], "U">,
             HelpText<"Undefine a symbol for the C preprocessor.">;
 
 def LANG_ID : Separate<[ "/", "-" ], "L">,