[STLExtras] Add size() for ranges, and remove distance()

[STLExtras] Add size() for ranges, and remove distance()

r332057 introduced distance() for ranges. Based on post-commit feedback,
this renames distance() to size(). The new size() is also only enabled
when the operation is O(1).

Differential Revision: https://reviews.llvm.org/D46976

llvm-svn: 332551