Official build break: symbol files not found, making indexing fail

[ravimeda]

https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_build?_a=summary&buildId=856273
Index symbols on Symweb task fails as shown below.

To fix this refer to dotnet/corefx#18350

2017-07-12T01:13:57.5914017Z ##[warning]Please stop using this task. See [Manage Your Symbols](https://aka.ms/MicroBuildSymbols) for guidance.
2017-07-12T01:13:57.8570074Z ##[error]\\cpvsbuild\drops\DotNetCore\DotNet-CoreRt-Master-4582_DotNet-CoreRT-Windows\20170712-01\symbols is not a valid symstore path; \\cpvsbuild\drops\DotNetCore\DotNet-CoreRt-Master-4582_DotNet-CoreRT-Windows\20170712-01\symbols\000Admin does not exist
2017-07-12T01:13:57.8726315Z ##[section]Finishing: Index Symbols on Symweb

[dagood]

Repro in these builds:
https://devdiv.visualstudio.com/DevDiv/_build?buildId=871160
https://devdiv.visualstudio.com/DevDiv/_build?buildId=871173

The root cause is that the Publish symbols path step found no files in $(Build.SourcesDirectory)\$(SourceFolder)\bin\Product for **\*.pdb, and didn't create the folder that the indexing step needed.

[MichalStrehovsky]

#4201 fixed it.