[libTooling] Add `getFileRange` as an alternative to `getRangeForEdit`

Add a `getFileRange` function alongside the existing `getRangeForEdit`
as a way to get a contiguous range within a single file (similar to
`getRangeForEdit`) but without the restriction that it cannot be in a
system header.

This can be used where a tool may want to use the range to extract the
source text. In such cases, we don't want to restrict this from
pulling from system headers.

Differential Revision: https://reviews.llvm.org/D141634