[clang][CoverageMapping] Refactor setting MC/DC True/False Condition IDs (#78202)

Clean-up of the algorithm that assigns MC/DC True/False control-flow
condition IDs when constructing an MC/DC decision region. This patch
creates a common API for setting/getting the condition IDs, making the
binary logical operator visitor functions much cleaner.

This patch also fixes issue
https://github.com/llvm/llvm-project/issues/77873 in which a record's
control flow map can be malformed due to an incorrect calculation of the
True/False condition IDs.