Use correct file extension for .tsx files being processed by LanguageService

[jkillian]

In noUnusedVariableRule.ts and noUseBeforeDeclareRule.ts, a LanguageServiceHost object is created and passed a hard-coded file name, file.ts. However, if the file we're working with is actually a .tsx file, it will be processed incorrectly by the language service. We need to detect what kind of file is being processed and use a correct extension.