SI-4270 Rename implicits to avoid name clashes.

This will fail implicit search in the next commit.
SI-4270 Disqualify in scope implicits that are shadowed.

If an expression wouldn't type check explicitly, it shouldn't be
allowed implicitly.

Employs typedIdent, which already does this sort of thing rather well,
instead of continuing to reimplement it in Implicits.

Remove check for non-implicit synonym, which is subsumed by typing an Ident.

Workaround Scaladoc oddity, by using an attributed select when the context
is deficient.
Don't crash if cpsParam is not on the classpath.

When checking if a piece of code needs the continuations plugin, the cpsParam classes may not be on the class path. Assume it does not need it in that case.
Don't use the BrowsingLoader for Java sources. 

When the presentation compiler needs a SourceLoader, instead of immediately adding the file 
to the compilation round it uses a fast parser to create symbols for all top-level class definitions. 
The fast parser assumes Scala sources, which is not always the case.
Don't require symbols to be loaded in the parser.

The parser special-cases primitive types. For comparing the name, it relied on having the symbols 
for primitive types already loaded, which is not always the case. The presentation compiler 
may parse sources for, say, scala.Int, and therefore the symbol for Int may not be yet available.
Remove NPE when `compileLate` sees sources during initialization.

When a symbol needed in Definitions is loaded, it may be that the corresponding source is newer than
the classfile. That will send it to `compileLate`, but no phase is yet set. We simply add it to the 
compilation pipeline and continue gracefully.
Refactoring and temporary commit before finishing the fix.
Merge branch 'master' into sbt-reflecton-fix
SI-4270 Refactor for efficiency and clarity.

Avoids allocation of the hashset used by shadowing checks when
searching in the implicit scope, and replaces three checks for
`if (isLocal)` with a sprinkling of polymorphism.
More fixups.
CPS: enable return expressions in CPS code if they are in tail position

Adds a stack of context trees to AnnotationChecker(s). Here, it is used to
enforce that adaptAnnotations will only adapt the annotation of a return
expression if the expected type is a CPS type.

The remove-tail-return transform is reasonably general, covering cases such as
try-catch-finally. Moreover, an error is thrown if, in a CPS method, a return
is encountered which is not in a tail position such that it will be removed
subsequently.
Remove stray comment
Fix for java parser edge case.

Empty statements are A-OK.  Closes SI-5910.  Review by @dragos.
Remove unused type parameter.

There should probably be a warning about non-overriding methods
which have unreferenced type parameters.
Replace context stack of AnnotationChecker with new mode for typing returns
Remove unneeded use of Tree#id
exposes extra tests for symbols as discussed on reflection meeting

review by @odersky
SI-5617 Better error message for "x overrides nothing".

"It looks like you're trying to override a method", notes Clippy.
SI-4989 Reject super.x if an intermediate class declares x abstract.

This is in line with Java's treatment. Without this, an AbstractMethodError
is thrown at runtime.
SI-4842 Forbid access to in-construction this in self-constructor args

The check was already in place for direct calls to the super constructor.
Without this additional check, ExplicitOuter crashes, as it doesn't create
an $outer pointer for the constructor-arg scoped inner object, but expects
one to exist when transforming the Outer.this reference.