[frontend] Input buffers have names too

There's no need to skip filename checks for in-memory buffers, and
indeed we want sourcekitd to behave the same for main.swift files as the
compiler.

SR-6624
rdar://36113282

Author: benlangmuir

lib/Frontend/Frontend.cpp

@@ -233,7 +233,7 @@ bool CompilerInstance::setUpForInput(const InputFile &input) {
     return false;
 
   if (isInSILMode() ||
-      (input.buffer() == nullptr && isInputSwift() &&
+      (isInputSwift() &&
        llvm::sys::path::filename(input.file()) == "main.swift")) {
     assert(MainBufferID == NO_SUCH_BUFFER && "re-setting MainBufferID");
     MainBufferID = *bufferID;

test/SourceKit/Sema/Inputs/empty.swift

@@ -0,0 +1 @@
+print("hi")

test/SourceKit/Sema/Inputs/top_level.swift

@@ -0,0 +1,11 @@
+// RUN: %sourcekitd-test -req=sema %s -- %s %S/Inputs/empty.swift -module-name main | %FileCheck %s -check-prefix=NO_ERROR
+// RUN: %sourcekitd-test -req=sema %s -- %s %S/Inputs/top_level.swift -module-name main | %FileCheck %s -check-prefix=NO_ERROR
+// RUN: %sourcekitd-test -req=sema %S/Inputs/empty.swift -- %s %S/Inputs/empty.swift -module-name main | %FileCheck %s -check-prefix=NO_ERROR
+// NO_ERROR-NOT: source.diagnostic.severity.error
+
+// RUN: %sourcekitd-test -req=sema %S/Inputs/top_level.swift -- %s %S/Inputs/top_level.swift -module-name main | %FileCheck %s -check-prefix=TOP_LEVEL_ERROR
+// TOP_LEVEL_ERROR: key.filepath: {{.*}}top_level.swift
+// TOP_LEVEL_ERROR-NEXT: key.severity: source.diagnostic.severity.error,
+// TOP_LEVEL_ERROR-NEXT: key.description: {{.*}}top level
+
+print("hi") // Top-level code.