[ModuleInterface] Intro export-as for Swift modules

Introduce a new flag `-export-as` to specify a name used to identify the
target module in swiftinterfaces. This provides an analoguous feature
for Swift module as Clang's `export_as` feature.

In practice it should be used when a lower level module `MyKitCore` is
desired to be shown publicly as a downstream module `MyKit`. This should
be used in conjunction with `@_exported import MyKitCore` from `MyKit`
that allows clients to refer to all services as being part of `MyKit`,
while the new `-export-as MyKit` from `MyKitCore` will ensure that the
clients swiftinterfaces also use the `MyKit` name for all services.

In the current implementation, the export-as name is used in the
module's clients and not in the declarer's swiftinterface (e.g.
`MyKitCore`'s swiftinterface still uses the `MyKitCore` module name).
This way the module swiftinterface can be verified. In the future, we
may want a similar behavior for other modules in between `MyKitCore` and
`MyKit` as verifying a swiftinterface referencing `MyKit` without it
being imported would fail.

rdar://103888618

lib/Serialization/ModuleFile.cpp

@@ -1324,3 +1324,10 @@ StringRef SerializedASTFile::getModuleDefiningPath() const {
 
   return moduleFilename;
 }
+
+StringRef SerializedASTFile::getExportedModuleName() const {
+  auto name = File.getModuleExportAsName();
+  if (!name.empty())
+    return name;
+  return FileUnit::getExportedModuleName();
+}