[CodeGen/Dwarf] Rename the "sizetype" synthetic type and add it to the accelerator table

Summary:
This type is created on-demand and used as the base type for array
ranges. Since it is "special", its construction did not go through the
createTypeDIE function and so it was never inserted into the accelerator
table, although it clearly belongs there.

I add an explicit addAccelType call to insert it into the table.

During review, we also decided to rename the type to something more
unique to avoid confusion in case the user has own "sizetype" type. The
new name for the type size __ARRAY_SIZE_TYPE__.

Reviewers: JDevlieghere, aprantl, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45445

llvm-svn: 329705