[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/AST/ASTDumper.cpp

@@ -631,7 +631,7 @@ namespace {
     void visitTypeAliasDecl(TypeAliasDecl *TAD) {
       printCommon(TAD, "typealias");
       PrintWithColorRAII(OS, TypeColor) << " type='";
-      if (auto underlying = TAD->getUnderlyingType()) {
+      if (auto underlying = TAD->getCachedUnderlyingType()) {
         PrintWithColorRAII(OS, TypeColor)
           << underlying.getString();
       } else {