[MSP430] Improve support of 'interrupt' attribute

* Accept as an argument constants in range 0..63 (aligned with TI headers and linker scripts provided with TI GCC toolchain).
* Emit function attribute 'interrupt'='xx' instead of aliases (used in the backend to create a section for particular interrupt vector).
* Add more diagnostics.

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D56663

llvm-svn: 351344