Conventionally, code completion results are matched with user input solely by
names. However, names are limited in expressiveness. From this comments, we start to
decorate code completion results with @keywords fields extracted from Clang doc comments.
These fields are added by API authors to comment the decl with information that
is not manifested clear enough through names. Code completion users' typing of the
keyword leads to the corresponding code completion results being selected as well.
Keywords can be arbitrarily long and can be multiple.
For instance, a function called "index()" has "@keyword find" in its doc comment.
Users' typing of "find" leads to "index()" being selected in the code completion list.
The code goes into its own sub-tree under 'tools' but tests go under 'test',
so that running 'check-swift' will also run all the SourceKit tests.
SourceKit is disabled on non-darwin platforms.
