[analyzer] Revise CStringChecker's modelling of strcpy() and strcat():

- (bounded copies) Be more conservative about how much is being copied.
- (str(n)cat) If we can't compute the exact final length of an append operation, we can still lower-bound it.
- (stpcpy) Fix the conjured return value at the end to actually be returned.

This requires these supporting changes:
- C string metadata symbols are still live even when buried in a SymExpr.
- "Hypothetical" C string lengths, to represent a value that /will/ be passed to setCStringLength() if all goes well. (The idea is to allow for temporary constrainable symbols that may end up becoming permanent.)
- The 'checkAdditionOverflow' helper makes sure that the two strings being appended in a strcat don't overflow size_t. This should never *actually* happen; the real effect is to keep the final string length from "wrapping around" in the constraint manager.

This doesn't actually test the "bounded" operations (strncpy and strncat) because they can leave strings unterminated. Next on the list!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133046 91177308-0d34-0410-b5e6-96231b3b80d8

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