Fix off-by-one error in #pragma clang system_header.

The system_header pragma (from GCC) is implemented using line notes in the
source manager. However, a line note's line number specifies the number
not for the current line, but for the next line. This was making all
line numbers appear off by one after the pragma.

Reported by Andy Gibbs, uncovered during r179677.

llvm-svn: 179709