[clang-format] Refactoring and asserts in LevelIndentTracker. (NFC)

adjustToUnmodifiedLine: The code does something only for non-PP-directives.
This is now reflected by putting the if-check to the top. This also ensures
that the assert() there is executed only if IndentForLevel is actually
accessed.

getIndent(): assert valid index into IndentForLevel.

Added explanation regarding the intention of IndentForLevel.

Differential Revision: https://reviews.llvm.org/D155094