The VFS tests were using Unix absolute paths, which does not play well
when Windows see them as relative to the current drive letter.
By using the temporal directory, both Windows and Unix can use the same
paths and avoid the problem.
Additionally, a couple of inputs have to be transformed into the native
path format, because sourcekitd-test compares the inputs as strings, and
they need to match exactly. So the source file and the name of the VFS
entries are transformed into native using the helper from LLVM support.
There were some changes to completion results because AST mutations that were
made while diagnosing are no longer happening.
This patch 1) changes expression type checking to allow unresolved types when
solving constraint systems, so we get a solution and apply its types in more
cases, and 2) fixes a parsing issue where we would drop a ternary expression
completely if the code completion point was in its true branch.
When filtering, make sure the FileCheck sees only the filtered results
and not accidentally pick up the unfiltered output from the initial
"open" request.
