If file is missing gets created still ok to use module resolutions since those are files from reference path resolution

Author: sheetalkamat

src/compiler/program.ts

@@ -1359,7 +1359,7 @@ namespace ts {
             // and hence cant reuse the structure.
             // This is same as how we dont reuse the structure if one of the file from old program is now missing
             if (oldProgram.getMissingFilePaths().some(missingFilePath => host.fileExists(missingFilePath))) {
-                return StructureIsReused.Not;
+                return StructureIsReused.SafeModules;
             }
 
             const newSourceFiles: SourceFile[] = [];

src/testRunner/unittests/reuseProgramStructure.ts

@@ -331,7 +331,7 @@ namespace ts {
             const program2 = updateProgram(program1, ["a.ts"], options, noop, newTexts);
             assert.lengthOf(program2.getMissingFilePaths(), 0);
 
-            assert.equal(program2.structureIsReused, StructureIsReused.Not);
+            assert.equal(program2.structureIsReused, StructureIsReused.SafeModules);
         });
 
         it("resolution cache follows imports", () => {

tests/baselines/reference/tscWatch/programUpdates/handles-the-missing-files---that-were-added-to-program-because-they-were-added-with-tripleSlashRefs.js

@@ -84,7 +84,7 @@ Output::
 
 Program root files: ["/a/b/commonFile1.ts"]
 Program options: {"watch":true}
-Program structureReused: Not
+Program structureReused: SafeModules
 Program files::
 /a/lib/lib.d.ts
 /a/b/commonFile2.ts