[Fixed Point Arithmetic] Fix for bug where integer literals could be treated as fixed...

[Fixed Point Arithmetic] Fix for bug where integer literals could be treated as fixed point literals

This addresses a bug brought up in https://bugs.llvm.org/show_bug.cgi?id=38161 where integer literals could be treated as fixed point types and throw errors related to fixed point types when the 'k' or 'r' suffix used. The fix also addresses the second issue brought up with the assertion by not treating integers as fixed point types in the first place.

Integers that have suffixes 'k' and 'r' now throw the error `invalid suffix 'k/r' on integer constant`.

A few more tests were also added to ensure that fixed point types, and any errors/warnings related to them, are limited to C for now.

Prior discussion also at https://reviews.llvm.org/D46915.

Differential Revision: https://reviews.llvm.org/D49327

llvm-svn: 337289