Uniformize MemRefType well-formedness checks.

Introduce a new public static member function, MemRefType::getChecked, intended
for the users that want detailed error messages to be emitted during MemRefType
construction and can gracefully handle these errors.  This function takes a
Location of the "MemRef" token if known.  The parser is one user of getChecked
that has location information, it outputs errors as compiler diagnostics.
Other users may pass in an instance of UnknownLoc and still have error messages
emitted.  Compiler-internal users not expecting the MemRefType construction to
fail should call MemRefType::get, which now aborts on failure with a generic
message.

Both "getChecked" and "get" call to a static free function that does actual
construction with well-formedness checks, optionally emits errors and returns
nullptr on failure.

The location information passed to getChecked has voluntarily coarse precision.
The error messages are intended for compiler engineers and do not justify
heavier API than a single location.  The text of the messages can be written so
that it pinpoints the actual location of the error within a MemRef declaration.

PiperOrigin-RevId: 219765902