Add implicit SPI import feature

API development sometimes requires a redesign while supporting early
adopters. Currently this is done by adding @_spi(name) to the API but
that requires adding the attribute in import statements as well, causing
manual overhead of adding and then removing when the redesign is done.

This PR introduces a special spi group name '_' and allows an implicit
spi import of a module containing API attributed with '@_spi(_)'

Resolves rdar://109797632

lib/Serialization/Serialization.cpp

@@ -2677,7 +2677,10 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
 
       SmallVector<IdentifierID, 4> spis;
       for (auto spi : theAttr->getSPIGroups()) {
-        assert(!spi.empty() && "Empty SPI name");
+        // SPI group name in source code can be '_', a specifier that allows
+        // implicit import of the SPI. It gets converted to to an empty identifier
+        // during parsing to match the existing AST node representation. An empty
+        // identifier is printed as '_' at serialization.
         spis.push_back(S.addDeclBaseNameRef(spi));
       }