Move get[S|U]LEB128Size() to LEB128.h.

This commit moves getSLEB128Size() and getULEB128Size() from
MCAsmInfo to LEB128.h and removes some copy-and-paste code.

Besides, this commit also adds some unit tests for the LEB128
functions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201937 91177308-0d34-0410-b5e6-96231b3b80d8