[Preprocessor] Ensure newline after #pragma introduced by -fms-extensions.

[Meinersbur]

Summary

The -fms-extensions converts __pragma (and _Pragma) into a #pragma that has to occur at the beginning of a line and end with a newline. This patch ensures that the newline after the #pragma is added even though that `Token::isAtStartOfLine() indicated that we should not start a newline.

[Meinersbur]

Refine test case

[Meinersbur]

• Use %clang_cc1 because %clang adds options according to it triple, i.e. -fms-extensions is added implicitly for Windows targets.

clang/lib/Frontend/PrintPreprocessedOutput.cpp

void PrintPPOutputPPCallbacks::HandleWhitespaceBeforeTok(const Token &Tok,
                                                          bool RequireSameLine) {
   // These tokens are not expanded to anything and don't need whitespace before
   // them.
   if (Tok.is(tok::eof) ||
       (Tok.isAnnotation() && !Tok.is(tok::annot_header_unit) &&
        !Tok.is(tok::annot_module_begin) && !Tok.is(tok::annot_module_end)))
     return;
 
-  if (!RequireSameLine && MoveToLine(Tok, /*RequireStartOfLine=*/false)) {
+  // EmittedDirectiveOnThisLine takes priority over RequireSameLine.
+  if ((!RequireSameLine || EmittedDirectiveOnThisLine) &&
+      MoveToLine(Tok, /*RequireStartOfLine=*/EmittedDirectiveOnThisLine)) {
     if (MinimizeWhitespace) {
       // Avoid interpreting hash as a directive under -fpreprocessed.
       if (Tok.is(tok::hash))
         OS << ' ';
     } else {
       // Print out space characters so that the first token on a line is
       // indented for easy reading.
       unsigned ColNo = SM.getExpansionColumnNumber(Tok.getLocation());

clang/test/Preprocessor/whitespace-ms-extensions.c

+// RUN: %clang -E -P %s -o -| FileCheck %s
+// RUN: %clang -E -P -fms-extensions %s -o -| FileCheck %s --check-prefix=MSEXT
+
+// -fms-extensions changes __pragma into #pragma
+// Ensure that there is a newline after the #pragma line.
+
+#define MACRO        \
+    text             \
+    __pragma(PRAGMA) \
+    after
+
+before MACRO text
+
+
+// CHECK:      before text __pragma(PRAGMA) after text
+
+// MSEXT:      before text
+// MSEXT-NEXT: #pragma PRAGMA
+// MSEXT-NEXT: after text