Graph based cache builds fail with cache aggregation error on isolation exempt shared projects

### Steps to reproduce

Do a graph based cache based build on the zipped repro:
[isolationExemptBug.zip](https://github.com/microsoft/msbuild/files/3806827/isolationExemptBug.zip)

Command line (using the test script from https://github.com/microsoft/MSBuildStaticGraphTester/blob/master/scripts/Test.ps1)
```
.\scripts\Build.ps1 -BuildRepos
.\scripts\Test.ps1 -singleProjectDirectory E:\delete\isolationExemptBug\ -singleProjectEntryFile E:\delete\isolationExemptBug\1\1.proj -includeMSBuildTests
```
### Expected behavior
Both builds succeed

### Actual behavior
The cache roundtrip test fails with
```
================================================================================================
=========================Cache roundtrip: E:\delete\isolationExemptBug\=========================
================================================================================================
Loading graph from solution E:\delete\isolationExemptBug\1\1.proj
Root Count: 1
Node Count: 3
Build started 11/4/2019 5:31:37 PM.
Using input build results caches:
Writing build results caches to: E:\delete\isolationExemptBug\\caches\3-54444047
__________________________________________________
Project "E:\delete\isolationExemptBug\3\3.proj" (Build target(s)):

Target Undeclared:
MSB4260: Project "E:\delete\isolationExemptBug\3\3.proj" skipped graph isolation constraints on referenced project "E:\delete\isolationExemptBug\4\4.proj"
__________________________________________________
Project "E:\delete\isolationExemptBug\3\3.proj" is building "E:\delete\isolationExemptBug\4\4.proj" (Build target(s)):

Target Build:
Hello 4

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:00.41
Build started 11/4/2019 5:31:38 PM.
Using input build results caches:
Writing build results caches to: E:\delete\isolationExemptBug\\caches\2-34640832
__________________________________________________
Project "E:\delete\isolationExemptBug\2\2.proj" (Build target(s)):

Target Undeclared:
MSB4260: Project "E:\delete\isolationExemptBug\2\2.proj" skipped graph isolation constraints on referenced project "E:\delete\isolationExemptBug\4\4.proj"
__________________________________________________
Project "E:\delete\isolationExemptBug\2\2.proj" is building "E:\delete\isolationExemptBug\4\4.proj" (Build target(s)):

Target Build:
Hello 4

Build succeeded.
0 Warning(s)
0 Error(s)

Time Elapsed 00:00:00.01
Build started 11/4/2019 5:31:38 PM.
Using input build results caches: E:\delete\isolationExemptBug\\caches\2-34640832;E:\delete\isolationExemptBug\\caches\3-54444047

Unhandled Exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: Input caches should not contain entries for the same configuration
at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args) in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Shared\ErrorUtilities.cs:line 70
at Microsoft.Build.Execution.CacheAggregator.InsertCaches(IConfigCache configCache, IResultsCache resultsCache) in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\CacheAggregator.cs:line 75
at Microsoft.Build.Execution.CacheAggregator.Aggregate() in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\CacheAggregator.cs:line 48
at Microsoft.Build.Execution.BuildManager.ReuseOldCaches(String[] inputCacheFiles) in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\BuildManager.cs:line 2461
at Microsoft.Build.Execution.BuildManager.g__InitializeCaches|53_1() in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\BuildManager.cs:line 477
at Microsoft.Build.Execution.BuildManager.BeginBuild(BuildParameters parameters) in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\BuildManager.cs:line 405
at Microsoft.Build.Execution.BuildManager.Build(BuildParameters parameters, BuildRequestData requestData) in E:\projects\MSBuildStaticGraphTester\repo\MSBuild\src\Build\BackEnd\BuildManager\BuildManager.cs:line 816
at msb.Program.BuildProject(String projectInstanceFullPath, IDictionary`2 globalProperties, IReadOnlyCollection`1 entryTargets, String[] inputCachesFiles, String outputCacheFile) in E:\projects\MSBuildStaticGraphTester\src\msb\Program.cs:line 411
at msb.Program.BuildGraphWithCacheFileRoundtrip(IReadOnlyCollection`1 projectFiles, String cacheRoot) in E:\projects\MSBuildStaticGraphTester\src\msb\Program.cs:line 332
at msb.Program.BuildWithCacheRoundtrip(IReadOnlyList`1 args) in E:\projects\MSBuildStaticGraphTester\src\msb\Program.cs:line 275
at msb.Program.Main(String[] args) in E:\projects\MSBuildStaticGraphTester\src\msb\Program.cs:line 141
```

### Probable cause
A project's reference can be excluded from graph isolation constraint checks by adding it to a `GraphIsolationExemptReference` item. When multiple projects exempt a shared project, the build results from the exempt project leak into the output results cache and cause a cache aggregation failure on a depending project.

For example, given the graph:
`1 -> 2, 3`
With projects 2 and 3 exempting an unknown-to-the-graph project `4`.

Both projects `2` and `3` will build `4`, and both will include `4`'s results in their caches. Project 1 then tries to aggregate the output caches from `2` and `3`, and blows up.

A potential fix could be to avoid writing `4`'s results to the results cache. It has been exempt from isolation constraints, and maybe it should be invisible to the build as well. The `GraphIsolationExemptReference` feature was introduced to exempt the build-time generated xaml projects, but those projects don't get shared, so everything was fine with them.