[analyzer] Fix use of length in CStringChecker

CStringChecker is using getByteLength to get the length of a string
literal. For targets where a "char" is 8-bits, getByteLength() and
getLength() will be equal for a C string, but for targets where a "char"
is 16-bits getByteLength() returns the size in octets.

This is verified in our downstream target, but we have no way to add a
test case for this case since there is no target supporting 16-bit
"char" upstream. Since this cannot have a test case, I'm asserted this
change is "correct by construction", and visually inspected to be
correct by way of the following example where this was found.

The case that shows this fails using a target with 16-bit chars is here.
getByteLength() for the string literal returns 4, which fails when
checked against "char x[4]". With the change, the string literal is
evaluated to a size of 2 which is a correct number of "char"'s for a
16-bit target.

```
void strcpy_no_overflow_2(char *y) {
  char x[4];
  strcpy(x, "12"); // with getByteLength(), returns 4 using 16-bit chars
}
```

This change exposed that embedded nulls within the string are not
handled. This is documented as a FIXME for a future fix.

```
    void strcpy_no_overflow_3(char *y) {
      char x[3];
      strcpy(x, "12\0");
    }

```

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D129269