[mlir] Making verification after parsing optional

[rriddle]

Summary

This is very useful when you want to parse IR even if
its invalid (e.g. bytecode). It's also useful if you don't
want to pay the cost of verification in certain situations.

[lattner]

Nice, thank you for the test

[bondhugula]

mlir/unittests/Parser/ParserTest.cpp

2

This is a header file's header. Drop "- C++ -".

[bzcheeseman]

Thanks river :)

[GleasonK]

Just noticed this config existed - will be very useful! Thanks!