`--forceConsistentCasingInFileNames` complains when given absolute paths with wrong casing

To reproduce (Windows only):

```
cd C:\
mkdir repro; cd repro
echo "export default 0" > a.ts
echo "import a from './a'" > b.ts
tsc c:\repro\a.ts c:\repro\b.ts --forceConsistentCasingInFileNames
```

The error is:

```
b.ts(1,15): error TS1149: File name 'C:/repro/a.ts' differs from already included file name 'c:/repro/a.ts' only in casing
```
