Use msbuild logging for symbol messages

[mmitche]

Because the symbol upload internally uses the msbuild loggers, if this code uses a string builder and then prints all the info at the end, the binlog tends to look like:

symbol upload diagnostic for symbol package 1
symbol upload diagnostic for symbol package 1
symbol upload diagnostic for symbol package 2
symbol upload diagnostic for symbol package 2
symbol upload diagnostic for symbol package 3
symbol upload diagnostic for symbol package 3
...
top level upload info for 1
top level upload info for 2
top level upload info for 3

Instead, it's easier to read as:

top level upload info for 1
symbol upload diagnostic for symbol package 1
symbol upload diagnostic for symbol package 1
top level upload info for 2
symbol upload diagnostic for symbol package 2
symbol upload diagnostic for symbol package 2
top level upload info for 3
symbol upload diagnostic for symbol package 3
symbol upload diagnostic for symbol package 3

There will be some interleaving due to parallelism, but right now it's difficult to line up the top level messages with the diagnostic ones.

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation

[mmitche]

@epananth I can wait till your stuff goes in to avoid merge conflicts.

[epananth]

Yes, Still building. Will merge it soon and let you know..

[epananth]

I merged my changes.