Target a single assignment in bounds checking error messages

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#893

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• In bounds checking related diagnostic messages, highlight the assignment within a top-level statement that causes the error or warning. This PR considers two types of bounds checking error messages for a variable V:
  1. The compiler cannot prove or can disprove the declared bounds for V are valid after an assignment to a variable in the bounds of V; and
  2. The inferred bounds of V become unknown after an assignment to a variable used in the declared bounds for V.
• Print 'assignment', 'increment', 'decrement' or 'initialization' in the error message depending on the "type" of the target expression.

An example

array_ptr<int> p : count(len) = ...;
other = 0, len = len * 2;

Error message before the change (highlight the whole statement):

error: inferred bounds for 'p' are unknown after statement
  other = 0, len = len * 2;
  ^~~~~~~~~~~~~~~~~~~~~~~~~

After the change (blame the embedded assignment that caused the error; print 'assignment' instead of 'statement'):

error: inferred bounds for 'p' are unknown after assignment
  other = 0, len = len * 2;
             ^~~~~~~~~~~~~