Introduce a new clang-cc option

  -remap-file=from;to

which takes the file "from" and transparently replaces its contents
with the contents of the file "to" from the source manager's
perspective. This is the moral equivalent of

  cp from saved
  cp to from
  <call clang>
  cp saved from
  rm saved

without all of the pesky file copying.

llvm-svn: 90307