[Lexer] NFC: Fix an off-by-one bug in getAsCharRange().

As the unit test demonstrates, subtracting 1 from the offset was unnecessary.

The only user of this function was the plist file emitter (in Static Analyzer
and ARCMigrator). It means that a lot of Static Analyzer's plist arrows
are in fact off by one character. The patch carefully preserves this
completely incorrect behavior and causes no functional change,
i.e. no plist format breakage.

Differential Revision: https://reviews.llvm.org/D59977

llvm-svn: 357823