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ARROW-7819: [C++][Gandiva] Add DumpIR to Filter/Projector object

The following patch exposes the generated IR as a method of the objects
for further inspection. This is a breaking change for the internal
method `FinalizeModule` which doesn't take the dump_ir and optimize
flags, it receives `optimize` from Configuration now.

- Refactored Engine, notably removed dead code, organized init in a single
  function and simplified LLVMGenerator.
- Dumping IR should not write to stdout, but instead return it as a
  string in the `DumpIR` method.
- Refactored Types, fixing some bad methods type.
- Added the optimize field to `Configuration` class.
- Simplified some unit tests.

But more importantly, we can now inspect dynamically:

```python
>>> filter = gandiva.make_filter(table.schema, condition)
>>> print(filter.ir)
; ModuleID = 'codegen'
source_filename = "codegen"
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@llvm.global_ctors = appending global [0 x { i32, void ()*, i8* }] zeroinitializer
@_ZN5arrow7BitUtilL8kBitmaskE = internal unnamed_addr constant [8 x i8] c"\01\02\04\08\10 @\80", align 1

; Function Attrs: norecurse nounwind
define i32 @expr_0_0(i64* nocapture readonly %args, i64* nocapture readonly %arg_addr_offsets, i64* nocapture readnone %local_bitmaps, i16* nocapture readnone %selection_vector, i64 %context_ptr, i64 %nrecords) local_unnamed_addr #0 {
entry:
  %0 = bitcast i64* %args to i8**
  %cond_mem56 = load i8*, i8** %0, align 8
  %1 = getelementptr i64, i64* %arg_addr_offsets, i64 3
  %2 = load i64, i64* %1, align 8
  %a_mem_addr = getelementptr i64, i64* %args, i64 3
  %3 = bitcast i64* %a_mem_addr to double**
  %a_mem7 = load double*, double** %3, align 8
  %scevgep = getelementptr double, double* %a_mem7, i64 %2
  br label %loop

loop:                                             ; preds = %loop, %entry
  %loop_var = phi i64 [ 0, %entry ], [ %"loop_var+1", %loop ]
  %scevgep8 = getelementptr double, double* %scevgep, i64 %loop_var
  %a = load double, double* %scevgep8, align 8
  %4 = fcmp olt double %a, 1.000000e+03
  %5 = sext i1 %4 to i8
```

Closes #6417 from fsaintjacques/ARROW-7819-gandiva-dump-ir-tool and squashes the following commits:

c8d274f98 <François Saint-Jacques> Address comments
0bcebc8b0 <François Saint-Jacques> ARROW-7819:  Add DumpIR to Filter/Projector object

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>