Add -no-toolchain-stdlib-rpath flag.

Add `-no-toolchain-stdlib-rpath` flag: the negative version of
`-toolchain-stdlib-rpath`.

Make `-no-toolchain-stdlib-rpath` be the default: use `/usr/lib/swift` as
default RPATH on Darwin platforms instead of toolchain standard library.

Adapted from swiftlang#27206.

tensorflow branch requires the opposite default (use toolchain standard
library as RPATH) because some stdlib modules like TensorFlow do not exist in
`/usr/lib/swift`.

Author: dan-zheng

include/swift/Option/Options.td

@@ -508,6 +508,9 @@ def no_static_stdlib: Flag<["-"], "no-static-stdlib">,
 def toolchain_stdlib_rpath: Flag<["-"], "toolchain-stdlib-rpath">,
   Flags<[HelpHidden,DoesNotAffectIncrementalBuild]>,
   HelpText<"Add an rpath entry for the toolchain's standard library, rather than the OS's">;
+def no_toolchain_stdlib_rpath: Flag<["-"], "no-toolchain-stdlib-rpath">,
+  Flags<[HelpHidden,DoesNotAffectIncrementalBuild]>,
+  HelpText<"Do not add an rpath entry for the toolchain's standard library, rather than the OS's">;
 def no_stdlib_rpath: Flag<["-"], "no-stdlib-rpath">,
   Flags<[HelpHidden,DoesNotAffectIncrementalBuild]>,
   HelpText<"Don't add any rpath entries.">;

lib/Driver/DarwinToolChains.cpp

@@ -389,7 +389,8 @@ toolchains::Darwin::addArgsToLinkStdlib(ArgStringList &Arguments,
     Arguments.push_back(context.Args.MakeArgString(path));
   }
 
-  if (context.Args.hasArg(options::OPT_toolchain_stdlib_rpath)) {
+  if (context.Args.hasFlag(options::OPT_toolchain_stdlib_rpath,
+                           options::OPT_no_toolchain_stdlib_rpath, false)) {
     // If the user has explicitly asked for a toolchain stdlib, we should
     // provide one using -rpath. This used to be the default behaviour but it
     // was considered annoying in at least the SwiftPM scenario (see