[C2X] N3007 Type inference for object definitions

This patches implements the auto keyword from the N3007 standard
specification.
This allows deducing the type of the variable like in C++:
```
auto nb = 1;
auto chr = 'A';
auto str = "String";
```
The list of statements which allows the usage of auto:

    * Basic variables declarations (int, float, double, char, char*...)
    * Macros declaring a variable with the auto type

The list of statements which will not work with the auto keyword:

    * auto arrays
    * sizeof(), alignas()
    * auto parameters, auto return type
    * auto as a struct/typedef member
    * uninitialized auto variables
    * auto in an union
    * auto as a enum type specifier
    * auto casts
    * auto in an compound literals

Differential Revision: https://reviews.llvm.org/D133289