Solver based results are fast within a function, where the `ASTContext`
can be re-used. But it is significantly slower than the AST based
results when outside of a function. Instead of using solver based as the
primary results, only use them as a fallback for when AST based fails.
Resolves rdar://108930110.
C++ atomic's fetch_sub returns the previous value, where we want to
check the new value. This was causing massive memory leaks in SourceKit.
For ThreadSafeRefCountedBase, just switch to the one in LLVM that's
already correct. We should move the VPTR one to LLVM as well and then
we can get rid of this header.
rdar://problem/27358273
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.
