Fix Lexer::ComputePreamble when MaxLines parameter is non-zero.

The function was only counting lines that included tokens and not empty lines,
but MaxLines (mainly initiated to the line where the code-completion point resides)
is a count of overall lines (even empty ones).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139085 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed