[CodeView] Fix off-by-one error in def range gap emission

Also fixes a much worse bug where we emitted the wrong gap size for the
def range uncovered by the test for this issue.

Fixes PR31726.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292949 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed