Setup SemanticDB tests

[olafurpg]

This commit updates the SemanticDB tests to assert that the output from
converting TASTy to SemanticDB is the same as the SemanticDB produced by
the semanticdb-scalac compiler plugin for Scala 2.12.7. Test failures
report a diff like this:

Test dotty.semanticdb.Tests.testExample failed: java.lang.AssertionError:
--- tasty
+++ scala2
-class Example {
-  val a: String = "1"
+class Example /*example/Example#*/  {
+  val a /*example/Example#a.*/ : String /*scala/Predef.String#*/  = "1"

The "tasty" output is currently identical to the original source file
because the TASTy to SemanticDB converter doesn't do anything.
Once the converter is updated to emit SymbolOccurrence
(https://scalameta.org/docs/semanticdb/specification.html#symboloccurrence)
then the "tasty" output will include inline comments like
class Example /* pkg/Example# */ that can be read as "the symbol
pkg/Example# was resolved next to the identifier on the left".
The spec for SemanticDB symbols can be found here:

• Scala symbols: https://scalameta.org/docs/semanticdb/specification.html#scala-symbol
• Java symbols: https://scalameta.org/docs/semanticdb/specification.html#java-symbol

To run the tests you must first produce the SemanticDB files for the
semanticdb/input project by running the following commands:

cd semanticdb/input
sbt compile

It's possible to include the semanticdb/input project in the main Dotty
build but I tried to limit the number of changes to the main build.

We use the org.scalameta:semanticdb_2.12 dependency to construct
SemanticDB data structures. If there is interest it's possible to
replace that dependency with a pure-Java module. I don't estimate it's a
lot of work, I have just never used the Java protobuf binding generator.

[dottybot]

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[nicolasstucki]

@olafurpg could you just add a README in the semanticdb folder with the instructions of how to run the tests. Otherwise it LGTM.

[olafurpg]

@nicolasstucki done #5268