[SourceKit] Reuse compiler instance between multiple completion

- Introduce ide::CompletionInstance to manage CompilerInstance
- `CompletionInstance` vends the cached CompilerInstance when:
-- The compiler arguments (i.e. CompilerInvocation) has has not changed
-- The primary file is the same
-- The completion happens inside function bodies in both previous and
   current completion
-- The interface hash of the primary file has not changed
- Otherwise, it vends a fresh CompilerInstance and cache it for the next
  completion

rdar://problem/20787086

tools/SourceKit/lib/SwiftLang/CodeCompletionOrganizer.cpp

@@ -58,7 +58,7 @@ class ImportDepth {
 
 public:
   ImportDepth() = default;
-  ImportDepth(ASTContext &context, CompilerInvocation &invocation);
+  ImportDepth(ASTContext &context, const CompilerInvocation &invocation);
 
   Optional<uint8_t> lookup(StringRef module) {
     auto I = depths.find(module);
@@ -320,7 +320,8 @@ CodeCompletionViewRef CodeCompletionOrganizer::takeResultsView() {
 // ImportDepth
 //===----------------------------------------------------------------------===//
 
-ImportDepth::ImportDepth(ASTContext &context, CompilerInvocation &invocation) {
+ImportDepth::ImportDepth(ASTContext &context,
+                         const CompilerInvocation &invocation) {
   llvm::DenseSet<ModuleDecl *> seen;
   std::deque<std::pair<ModuleDecl *, uint8_t>> worklist;