SystemTools: Fix GetActualCaseForPath drive letter case handling

Convert the drive letter to upper case in the actual output value
instead of just our internal memoization map key.  This was broken by
commit 08b2dcff (Fix GetActualCaseForPath for UNC paths, 2010-08-16).

CMake-Issue: 15642
Change-Id: Iee7148616370d1be4be052c74b7a5d802ad47d93