[AST] Remove SourceFile::addImports

Temporarily replace with `SourceFile::setImports`
until import resolution is requestified. Now
imports are only set once for a given SourceFile.

Because we're now asserting in more places that
import resolution must have run before querying
imports, this commit also adds
`getCachedUnderlyingType` to TypeAliasDecl to stop
the ASTDumper from trying to query imports for a
-dump-parse invocation.

lib/Sema/ImportResolution.cpp

@@ -288,7 +288,7 @@ void swift::performImportResolution(SourceFile &SF) {
   for (auto D : SF.getTopLevelDecls())
     resolver.visit(D);
 
-  SF.addImports(resolver.getFinishedImports());
+  SF.setImports(resolver.getFinishedImports());
 
   SF.ASTStage = SourceFile::ImportsResolved;
   verify(SF);