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.

llvm-svn: 29540