Option: add -libc as an option for Swift tools

Ensure that we process `-libc` in `swift-symbolgraph-extract` and
`swift-api-extract`.  This option is used by Windows to determine the C
ABI to use and thus impacts the ABI exposed by the ClangImporter to the
Swift interface.  This partially enables the use of
`swift package dump-symbol-graph` on Windows.

Author: compnerd

include/swift/Option/Options.td

@@ -834,7 +834,9 @@ def enable_app_extension : Flag<["-"], "application-extension">,
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Restrict code to those available for App Extensions">;
 
-def libc : Separate<["-"], "libc">, HelpText<"libc runtime library to use">;
+def libc : Separate<["-"], "libc">,
+           Flags<[SwiftAPIExtractOption, SwiftSymbolGraphExtractOption]>,
+           HelpText<"libc runtime library to use">;
 
 // Linker options