Merge pull request #15752 from Microsoft/PortCompileOnSaveFixToRelease-2.3

Port Compile On Save Fix to Release-2.3

Author: jramsay

src/services/shims.ts

@@ -1038,7 +1038,11 @@ namespace ts {
             return this.forwardJSONCall(`resolveModuleName('${fileName}')`, () => {
                 const compilerOptions = <CompilerOptions>JSON.parse(compilerOptionsJson);
                 const result = resolveModuleName(moduleName, normalizeSlashes(fileName), compilerOptions, this.host);
-                const resolvedFileName = result.resolvedModule ? result.resolvedModule.resolvedFileName : undefined;
+                let resolvedFileName = result.resolvedModule ? result.resolvedModule.resolvedFileName : undefined;
+                if (result.resolvedModule && result.resolvedModule.extension !== Extension.Ts && result.resolvedModule.extension !== Extension.Tsx && result.resolvedModule.extension !== Extension.Dts) {
+                    resolvedFileName = undefined;
+                }
+
                 return {
                     resolvedFileName,
                     failedLookupLocations: result.failedLookupLocations