[IDE, SourceKit] Pass in clang as arg[0] in initInvocationByClangArguments

This applies the same fix as cfdb8ac, but this time to libIDE so
that SourceKit and swift-ide-test will also pass in an appropriate
argv[0]. Re-enable the tests that broke.

rdar://problem/24431137

Author: benlangmuir

lib/IDE/Utils.cpp

@@ -238,9 +238,15 @@ bool ide::initInvocationByClangArguments(ArrayRef<const char *> ArgList,
                                                &DiagBuf,
                                                /*ShouldOwnClient=*/false);
 
+  // Clang expects this to be like an actual command line. So we need to pass in
+  // "clang" for argv[0].
+  std::vector<const char *> ClangArgList;
+  ClangArgList.push_back("clang");
+  ClangArgList.insert(ClangArgList.end(), ArgList.begin(), ArgList.end());
+
   // Create a new Clang compiler invocation.
   llvm::IntrusiveRefCntPtr<clang::CompilerInvocation> ClangInvok{
-    clang::createInvocationFromCommandLine(ArgList, ClangDiags)
+    clang::createInvocationFromCommandLine(ClangArgList, ClangDiags)
   };
   if (!ClangInvok || ClangDiags->hasErrorOccurred()) {
     for (auto I = DiagBuf.err_begin(), E = DiagBuf.err_end(); I != E; ++I) {

test/IDE/print_clang_header.swift

@@ -1,6 +1,5 @@
 // FIXME: rdar://problem/19648117 Needs splitting objc parts out
 // XFAIL: linux
-// REQUIRES: OS=macosx
 
 // RUN: echo '#include "header-to-print.h"' > %t.m
 // RUN: %target-swift-ide-test -source-filename %s -print-header -header-to-print %S/Inputs/print_clang_header/header-to-print.h -print-regular-comments --cc-args %target-cc-options -isysroot %clang-importer-sdk-path -fsyntax-only %t.m -I %S/Inputs/print_clang_header > %t.txt

test/IDE/print_clang_header_swift_name.swift

@@ -4,7 +4,6 @@
 // RUN:     -isysroot %clang-importer-sdk-path -fsyntax-only %t.m -I %S/Inputs | FileCheck %s
 
 // REQUIRES: objc_interop
-// REQUIRES: OS=macosx
 
 // CHECK: enum Normal : Int {
 // CHECK-NOT: {{^}}}