[NFC] Clean Up FrontendTool

Try to impose a simple structure that splits performing actions from the
pre and post-pipeline conditions. Wherever actions would take more than
a simple return, split them into functions. Refine functions that
perform effects to return status codes when they fail. Finally,
delineate functions that need semantic analysis from those that do not.

Overall this should be NFC.

Author: CodaFi

include/swift/Frontend/Frontend.h

@@ -621,7 +621,7 @@ class CompilerInstance {
   ///
   /// This is similar to a parse-only invocation, but module imports will also
   /// be processed.
-  void performParseAndResolveImportsOnly();
+  bool performParseAndResolveImportsOnly();
 
   /// Performs mandatory, diagnostic, and optimization passes over the SIL.
   /// \param silModule The SIL module that was generated during SILGen.

lib/Frontend/Frontend.cpp

@@ -848,7 +848,7 @@ void CompilerInstance::setMainModule(ModuleDecl *newMod) {
   Context->addLoadedModule(newMod);
 }
 
-void CompilerInstance::performParseAndResolveImportsOnly() {
+bool CompilerInstance::performParseAndResolveImportsOnly() {
   FrontendStatsTracer tracer(getStatsReporter(), "parse-and-resolve-imports");
 
   // Resolve imports for all the source files.
@@ -867,6 +867,7 @@ void CompilerInstance::performParseAndResolveImportsOnly() {
   mainModule->setHasResolvedImports();
 
   bindExtensions(*mainModule);
+  return Context->hadError();
 }
 
 void CompilerInstance::performSema() {