Unlike other prefix operators, unary minus is folded into the
NumberLiteralExpr in the parser. This commit recreates this effect in
the lexer-based syntax map so that token ranges will include the leading
minus.
rdar://problem/20205885
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.
