Make the removal of files use Path::eraseFromDisk just like it does for

Make the removal of files use Path::eraseFromDisk just like it does for
the removal of directories. Using std::remove is indiscriminate and can
lead to the removal of things like /dev/null if run as root. The
Path::eraseFromDisk method ensures that we only ever remove regular files
or directories, but never character or block special nodes.  This should
clear up the problem with usage like: llvm-as -o /dev/null which is used
in the llvm-test makefiles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29540 91177308-0d34-0410-b5e6-96231b3b80d8