SI-6138 Centralize and refine detection of `getClass` calls

`getClass` is special cased in the compiler; this is described
in in the comments on `Definitions.Any_getClass`.

Part of this happens in `Typer#stabilize`. This was trying to determine
if an Ident or Select node was a call to `getClass` by merits of the name
of the tree's symbol and by checking that the its type (if it was a
MethodType or PolyType) had no parameters in the primary parameter list.

Overloaded user defined `getClass` methods confused this check. In the
enclosed test case, the tree `definitions.this.getClass` had an
`OverloadedType`, and such types always report an empty list of `params`.

This commit:

  - changes `stabilize` to use `isGetClass`, rather than the
    homebrew check
  - changes `isGetClass` to consider a `Set[Symbol]` containing all
    `getClass` variants. This moves some similar code from `Erasure`
    to `Definitions`
  - keeps a fast negative path in `isGetClass` based on the symbol's name

1 files changed, 7 insertions, 0 deletions

diff --git a/test/files/neg/t6138.check b/test/files/neg/t6138.check
new file mode 100644
index 0000000000..8fd9978248
--- /dev/null
+++ b/test/files/neg/t6138.check
@@ -0,0 +1,7 @@
+t6138.scala:4: error: ambiguous reference to overloaded definition,
+both method getClass in object definitions of type (s: Int)Any
+and  method getClass in object definitions of type (s: String)Any
+match argument types (Nothing)
+  getClass(???): String
+  ^
+one error found