[RenderScript] Improve file format for saving RS allocations

Updates the file format for storing RS allocations to a file, so that the format now supports struct element types.

The file header will now contain a subheader for every RS element and it's descendants. 
Where an element subheader contains element type details and offsets to the subheaders of that elements fields. 

Patch also improves robustness when loading incorrect files.

llvm-svn: 257045