[Lexer] Fix an assertion failure when invoking code completion inside a single-quoted string literal

[ahoppen]

• Explanation: When performing code completion inside a single-quoted string literal, we were trying to generate a Fix-It to change the string literal to double quotes. This Fix-It accidentally contained the null byte that signified the code completion location, resulting in an assertion failure. Exclude the null byte from the Fix-It to resolve the assertion failure.
• Scope: Completion inside single-quoted string literals
• Risk: Fairly low, we are just skipping over null bytes in Fix-Its
• Testing: Added a regression test
• Issue: rdar://126924962
• Reviewer: @rintaro on [Lexer] Fix an assertion failure when invoking code completion inside a single-quoted string literal #73467

[ahoppen]

@swift-ci Please test