[MSP430] Allow msp430_intrcc functions to not have interrupt attribute.

Summary:
Useful in case you want to have control over interrupt vector generation.
For example in Rust language we have an arrangement where all unhandled
ISR vectors gets mapped to a single default handler function. Which is
hard to implement when LLVM tries to generate vectors on its own.

Reviewers: asl, krisb

Subscribers: hiraditya, JDevlieghere, awygle, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67313

llvm-svn: 372910