Refactoring: separate the cppcheck-specific &= handling from TokenList::combineOperators() #737

TokenList::combineOperators() joins single-character tokens into operators (& = → &=). It also contains a heuristic for
cppcheck: in a function declaration with an unnamed reference parameter that has a default value, &= is not joined.

void f(x&=2);          // simplecpp: void f ( x & = 2 ) ;
f(x&=2);               // simplecpp: f ( x &= 2 ) ;

This is not part of preprocessing. Compilers tokenize &= as one operator here (g++: expected ',' or '...' before '&=' token). To
decide, the heuristic tracks whether the code is inside a function body (executableScope) and looks back for a function
declaration. That is C++ parsing.

Before combineOperators() is replaced by a longest-match tokenizer (step 2 of #708), move this code into separate functions with
clear names, so that it can be kept as its own step:

• tracking of { / } to know if the current code is in a function body
• detection of "&= in the parameter list of a function declaration"

This is a pure refactoring, so behaviour does not change. The existing tests in combineOperators_andequal keep passing unchanged.