[Dexter] Add DexDeclareFile command to Dexter

DexDeclareFile allows test producers to write test files with .dex extensions
that contain pure dexter commands.

.dex file commands do not need to be commented out like they do when written
inline within test source files.

DexDeclareFile commands are declarative in behaviour, they state that any
Dexter command seen from this point on will have its path attribute set to the
path declared in the DexDeclareFile command.

Differential Revision: https://reviews.llvm.org/D99651