Handle init statements in readability-else-after-return

Adds a new ASTMatcher condition called 'hasInitStatement()' that matches if,
switch and range-for statements with an initializer. Reworked clang-tidy
readability-else-after-return to handle variables in the if condition or init
statements in c++17 ifs. Also checks if removing the else would affect object
lifetimes in the else branch.

Fixes PR44364.