[ValueTracking] Add CharWidth argument to getConstantStringInfo (NFC)
The method assumes that host chars and target chars have the same width.
Add a CharWidth argument so that it can bail out if the requested char
width differs from the host char width.
Alternatively, the check could be done at call sites, but this is more
error-prone.
In the future, this method will be replaced with a different one that
allows host/target chars to have different widths. The prototype will
be the same except that StringRef is replaced with something that is
byte width agnostic. Adding CharWidth argument now reduces the future
diff.