Build warnings -- cleanup fallthrough warnings

[henrygab]

Currently three files use the following to disable this warning on a couple functions:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
// ... code that generates warnings ...
#pragma GCC diagnostic pop

That was done as a temporary measure in #207 to allow finding/fixing other issues in the code exposed by compiling with -Wall.

Here's a list of correct ways to fix these implicit fallthrough warnings:

• For C++17 and higher, use [[fallthrough]] attribute
• For GCC at C++11 / C++14, use [[gnu::fallthrough]]
• For CLANG at C++11 / C++14, use [[clang::fallthrough]] attribute
• For GCC with older compilers, use __attribute__(( fallthrough ))

NOTE: The comment method is legacy and very fragile. See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78497

If code will be compiled by multiple compilers / versions, consider defining a macro such as ADAFRUIT_FALLTHROUGH, that conditionally defines to C++17, GCC, or CLANG as available.

[hathach]

there is no need for ADAFRUIT_FALLTHROUGH just either use attribute fallthru or [[fallthrough]]. We only need to fix current gcc and worry about other compiler later on when needed.

[hathach]

I really like attribute(( fallthrough )); It should be able to work with existing old & new code, just use it in another library, works like a charm

https://github.com/adafruit/Adafruit_SPIFlash/pull/70/files#diff-0d585c34005499efb9fdee8db227e2eaR111

[henrygab]

I agree that is a preferred attribute. Now the difficulty is in verifying that the code is actually doing the "right thing" in each case, before using this attribute. Else, the attribute will hide a true bug from smart compiler warnings.

[hathach]

@henrygab yeah, but I don't think we could find anyone that could insert it into the "right place" any time soon :(. Let's leave it as it is for now :)