[LLVM][IR] Switch from manual pointer incrementation to function in Lexer

[Bertik23]

The IR Lexer has a getNextChar() function implemented. But it is used only once, on other places the pointer pointing to the current position in the lexed string (CurPtr) is incremented manually, this can lead to some OOB errors (see #151556 and Discourse thread).

To mitigate the issue, this PR changes the getNextChar() to be more straight forward, changes all manual pointer increments to getNextChar() or adds checks, so that no OOB occurs.

Two helper functions were used skipNChars and advancePositionTo. Where skipNChars(N) calls getNextChar() N-times, while advancePositionTo(Ptr) is used insead of CurPtr = Ptr.

One helper function isLabelTail was moved into the Lexer class, to be able to check the bounds, and renamed to getLabelTail to better reflect it's function.

[Bertik23]

Ping @nikic as I haven't found a IR specific maintainer