Fix incorrectly finding 'executable' directories instead of files.

This broke for example the 'not' utility, if a directory called
'FileCheck' is executable and in the path before the actual 'FileCheck'.

This patch steals the implementation of the "old" PathV1 canExecute
implementation:
- checks for R_OK (file readable): this is necessary for executing
  scripts; we should not regress here unless we have good reasons
- checks for S_ISREG; if we want to get rid of this, we'd need to
  change all callers who already made the assumption when depending
  on Path V1.

llvm-svn: 184074