[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