[mlir] Floating constants for import-llvm

Summary:
`mlir-translate -import-llvm test.ll`  was going into segmentation fault if `test.ll` had `float` or `double` constants.
For example,
```
%3 = fadd double 3.030000e+01, %0
```
Now, it is handled in `Importer::getConstantAsAttr` (similar behaviour as normal integers)
Added tests for FP arithmetic

Reviewers: ftynse, mehdi_amini

Reviewed By: ftynse, mehdi_amini

Subscribers: shauheen, mehdi_amini, rriddle, jpienaar, burmako, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71912