[SAoC] MLIR Support for LDC

[Roberto Rosmaninho]

Hi everyone, I'm here for the last update of this 1st Milestone!

So, this week I refactored the code! Unfortunately, I wrote a 
bunch of useless code while I was trying to understand how ldc 
does some things, but that ok...the code is much better readable 
now, so if you have some interest in review the code I'm still 
updating this branch:
https://github.com/Robertorosmaninho/ldc/tree/fix-llvm-10

Since my last update I worked on the following tasks:
  - Refactoring the code.
     - I was doing some weird stuff to get access to all 
statements, so now it's possible to declare more than 1 function 
on a program.
     - I changed the name of the functions for the mangled name.
     - I had some problems with function parameters as well, but 
finally, I could fix.
  - I made mlir less verbose (HUGE improvements here!!)
  - Implement calls
  - Implement one-dimensional tensors (as a static array)

So far my implementation can deal with:
  - Simple VarDeclarations
  - Operations: +, - , x, /
  - Declare undimensional vectors
  - Function calls
  - All types are integer except the output of arrays

Issues:
  - MLIR is growing too fast I am more than 150 commits behind 
HEAD and at least yesterday wasn't compiling on my Linux with 
LLVM trunk.
  - LLVM trunk changed some stuff and I wasn't able to compile 
with my version of MLIR.

Where I'm at each project:
- LLVM: commit f79d8a064ce4d8846377e4abbc9a389b62f90d43 |  Wed 
Aug 28 15:40:34 2019.
- MLIR: I'm working on fix-llvm-10 branch so I'm behind commit 
b50775af52281d9f38aae6c359b72dc715408df0 of Aug 30.

This should be fixed next week, I will try to port my code to the 
next stable version of each project.

For the next steps:
  - Make CMake FindMLIR more general, it seems like it's not 
possible to reach projects directory by LLVM_ROOT_DIR on Linux so 
I have to figure out a new way to find MLIR.
  - Extend the size of tensor and work on support for all types...
  - Create the LDC Dialect