[STLExtras] Add a default value to drop_begin

This patch adds the default value of 1 to drop_begin.

In the llvm codebase, 70% of calls to drop_begin have 1 as the second
argument.  The interface similar to with std::next should improve
readability.

This patch converts a couple of calls to drop_begin as examples.

Differential Revision: https://reviews.llvm.org/D94858