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

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 if
Token::isAtStartOfLine() indicated that we should not start a newline.

Committing relying post-commit review since the change is small, some
downstream uses might be blocked without this fix, and to make clear the
decision of the new -fminimize-whitespace feature (fix on main, revert
on clang-13.x branch) suggested by @aaron.ballman in D104601.

Differential Revision: https://reviews.llvm.org/D107183

clang/lib/Frontend/PrintPreprocessedOutput.cpp

@@ -646,7 +646,9 @@ void PrintPPOutputPPCallbacks::HandleWhitespaceBeforeTok(const Token &Tok,
        !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))

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

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -E -P %s -o - | FileCheck %s
+// RUN: %clang_cc1 -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