Include memory usage in -diagnostics report

The `-diagnostics` command line switch now includes memory usage in the reported statistics when running under node.js. If node.js was invoked with the `-expose-gc` flag, `global.gc()` is called before obtaining the allocated heap size to get a more accurate number. For example:
```typescript
node -expose-gc tsc -diagnostics myProgram.ts
```
Note that memory usage is not reported with the cscript.exe host since it provides no way to obtain memory statistics.