Created a branded type for identifier-escaped strings (#16915)

* Created a branded type for escaped strings

Then flowed it throughout the compiler, finding and fixing a handful of
bugs relating to underscore-prefixed identifiers in the process.
Includes a test for two cases noticed - diagnostics from conflicting
symbols from export *'s, and enum with underscore prefixed member emit.

* Correctly double underscores WRT mapped types

* Add fourslash tests for other fixed issues

* use function call over cast

* Update forEachEntry type accuracy

* Just use escaped names for ActiveLabel

* Remove casts from getPropertyNameForPropertyNameNode

* This pattern has occurred a few times, could use a helper function.

* Remove duplicated helper

* Remove unneeded check, use helper

* Identifiers list is no longer escaped strings

* Extract repeated string-getting code into helper

* Rename type and associated functions

* Make getName() return UnderscoreEscapedString, add getUnescapedName()

* Add list of internal symbol names to escaped string type to cut back on casting

* Remove outdated comments

* Reassign interned values to nodes, just in case

* Swap to string enum

* Add deprecated aliases to escapeIdentifier and unescapeIdentifier

* Add temp var

* Remove unsafe casts

* Rename escaped string type as per @sandersn's suggestion, fix string enum usages

* Reorganize double underscore tests

* Remove jfreeman from TODO

* Remove unneeded parenthesis

src/compiler/program.ts

@@ -405,7 +405,7 @@ namespace ts {
         let commonSourceDirectory: string;
         let diagnosticsProducingTypeChecker: TypeChecker;
         let noDiagnosticsTypeChecker: TypeChecker;
-        let classifiableNames: Map<string>;
+        let classifiableNames: UnderscoreEscapedMap<__String>;
         let modifiedFilePaths: Path[] | undefined;
 
         const cachedSemanticDiagnosticsForFile: DiagnosticCache = {};
@@ -580,7 +580,7 @@ namespace ts {
             if (!classifiableNames) {
                 // Initialize a checker so that all our files are bound.
                 getTypeChecker();
-                classifiableNames = createMap<string>();
+                classifiableNames = createUnderscoreEscapedMap<__String>();
 
                 for (const sourceFile of files) {
                     copyEntries(sourceFile.classifiableNames, classifiableNames);