[COFF] Error out if 20 bit thumb branches are out of range

This is similar to what was added in SVN r277838 for 24 bit
branch instructions.

Differential Revision: https://reviews.llvm.org/D41163

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320677 91177308-0d34-0410-b5e6-96231b3b80d8