[analyzer] Deduplicate _Info* instances

When having several contexts we'll end up having many copies of the same
_Info* instances. This CL deduplicates it.
TL;DR: On flutter/flutter (82 contexts) this, when run from a clean
cache, reduces the memory usage by 600-800 MB (~15%).

Details:

All numbers from runs made from analyzer source with a clean cache on
flutter/flutter (82 contexts), memory usage after a forced GC:

Runtime:
No difference proven at 95.0% confidence

Current memory (GB):
Difference at 95.0% confidence
        -0.683333 +/- 0.0627592
        -13.3987% +/- 1.23057%
        (Student's t, pooled s = 0.0276887)

Peak memory (GB):
Difference at 95.0% confidence
        -0.643333 +/- 0.0746028
        -12.5243% +/- 1.45236%
        (Student's t, pooled s = 0.032914)

Heap (used) (GB):
Difference at 95.0% confidence
        -0.77 +/- 0
        -17.5799% +/- 0%
        (Student's t, pooled s = 0)

Heap (capacity) (GB):
Difference at 95.0% confidence
        -0.69 +/- 0.05552
        -15.2993% +/- 1.23104%
        (Student's t, pooled s = 0.0244949)

_List (MB):
Difference at 95.0% confidence
        -59.2333 +/- 0.0925333
        -6.91547% +/- 0.0108032%
        (Student's t, pooled s = 0.0408248)

_Uint32List (MB):
Difference at 95.0% confidence
        -105.433 +/- 1.06715
        -17.3382% +/- 0.175489%
        (Student's t, pooled s = 0.470815)

_OneByteString (MB):
Difference at 95.0% confidence
        -403.667 +/- 2.37542
        -58.1206% +/- 0.342017%
        (Student's t, pooled s = 1.04801)

Raw data:

NOW:

Runtime (ms): 151855
current memory	4.46GB
peak memory	4.54GB
heap 3.61GB of 3.86GB
797.3 MB 6067774 _List
503.1 MB 2574937 _Uint32List
291.7 MB 2233396 _OneByteString
233.0 MB 338029 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901830 _Map

Runtime (ms): 152410
current memory	4.40GB
peak memory	4.49GB
heap 3.61GB of 3.80GB
797.3 MB 6067637 _List
503.0 MB 2574976 _Uint32List
290.7 MB 2232326 _OneByteString
226.8 MB 339447 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901830 _Map

Runtime (ms): 153205
current memory	4.39GB
peak memory	4.45GB
heap 3.61GB of 3.80GB
797.3 MB 6067638 _List
501.9 MB 2556644 _Uint32List
290.2 MB 2235718 _OneByteString
226.7 MB 338584 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901830 _Map




BEFORE:

Runtime (ms): 159178
current memory	5.09GB
peak memory	5.15GB
heap 4.38GB of 4.51GB
856.6 MB 7366027 _List
696.0 MB 3780593 _OneByteString
608.1 MB 5239817 _Uint32List
226.9 MB 341102 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901829 _Map

Runtime (ms): 153913
current memory	5.11GB
peak memory	5.13GB
heap 4.38GB of 4.51GB
856.5 MB 7366249 _List
693.8 MB 3730773 _OneByteString
608.1 MB 5239925 _Uint32List
233.0 MB 338205 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901829 _Map

Runtime (ms): 157729
current memory	5.10GB
peak memory	5.13GB
heap 4.38GB of 4.51GB
856.5 MB 7366221 _List
693.8 MB 3730773 _OneByteString
608.1 MB 5239924 _Uint32List
233.0 MB 337860 _Uint8List
158.7 MB 3466691 Reference
116.1 MB 1901829 _Map

Change-Id: I5697ff0efd40c9325f7f15d8092655b80b4876ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/318940
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>