[SourceKit] Disable expensive SIL options (#17306)

Disable the sanitizers and code coverage when building a swift
invocation for the purpose of collecting diagnostics.

This should speed up diagnostic generation and reduce exposure to
compiler bugs.

rdar://40955900

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h

@@ -39,6 +39,7 @@ namespace swift {
   class Type;
   class AbstractStorageDecl;
   class SourceFile;
+  class SILOptions;
   class ValueDecl;
   enum class AccessorKind;
 
@@ -532,6 +533,10 @@ public:
   ~CloseClangModuleFiles();
 };
 
+
+/// Disable expensive SIL options which do not affect indexing or diagnostics.
+void disableExpensiveSILOptions(swift::SILOptions &Opts);
+
 } // namespace SourceKit
 
 #endif