[analyzer] Use sufficiently large types for index bounds calculation.

The ProgramState::assumeInBound() API is used by checkers to make an assumption
that a certain array index is within the array's bounds (i.e. is greater than or
equal to 0 and is less than the length of the array). When the type of the
index was unspecified by the caller, it assumed that the type is 'int', which
caused some indices and sizes to truncate during calculations.

Use ArrayIndexTy by default instead, which is used by the analyzer to represent
index types and is currently hardcoded to long long.

Patch by Bevin Hansson!

Differential Revision: https://reviews.llvm.org/D46944

llvm-svn: 335803