`llvm::drop_begin` and `llvm::drop_end` are inconsistently named

`ADT/STLExtras.h` provides `drop_begin` and `drop_end` functions to drop the first N or last N elements from a range, respectively. These names are inconsistent with `ArrayRef` and our general STL conventions, and should instead be named `drop_front` and `drop_back`.

Start in ADT/STLExtras.h, where llvm::drop_begin and drop_end are provided, and compare their naming with ArrayRef and the project’s general STL conventions. Search for references to both functions and update the naming consistently; done means the replacements use drop_front and drop_back without leaving the inconsistent names behind.

Written by the indexing model from the issue text.