[SourceKit] Use "-" as fallback complier argument

Since compiler arguments are processed by Driver, arbitrary filename
passed as 'request.name' (e.g. "", "-1", etc.) may cause problems.
Using '-' guarantees successful initialization of 'CompilerInvocation'.

rdar://problem/40646921
rdar://problem/40955808

Author: rintaro

test/SourceKit/DocumentStructure/structure.swift.placeholders.response

@@ -9,7 +9,7 @@
       key.name: "<#MyCls#>",
       key.offset: 0,
       key.length: 35,
-      key.runtime_name: "_TtC12placeholders9<#MyCls#>",
+      key.runtime_name: "_TtC4main9<#MyCls#>",
       key.nameoffset: 6,
       key.namelength: 9,
       key.bodyoffset: 34,

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1757,8 +1757,11 @@ void SwiftEditorDocument::parse(ImmutableTextSnapshotRef Snapshot,
     Impl.SemanticInfo->getInvocation()->applyTo(CompInv);
     Impl.SemanticInfo->getInvocation()->raw(Args, PrimaryFile);
   } else {
+    // Use stdin as a .swift input to satisfy the driver. Note that we don't
+    // use Impl.FilePath here because it may be invalid filename for driver
+    // like "" or "-foobar".
     SmallVector<const char *, 1> Args;
-    Args.push_back(Impl.FilePath.c_str()); // Input
+    Args.push_back("-");
     std::string Error;
     // Ignore possible error(s)
     Lang.getASTManager().