[clang-format] Respect definition separators when MaxEmptyLinesToKeep: 0 (#206406)

Fixes #206340.

The formatter should remove empty lines before Allman opening braces,
but should preserve the required empty line between function
definitions.

I have added a test case "AlwaysMaxEmptyLinesZeroAllman", and have
ensured other test cases run fine along with this one.

clang/lib/Format/DefinitionBlockSeparator.cpp

diff --git a/clang/lib/Format/DefinitionBlockSeparator.cpp b/clang/lib/Format/DefinitionBlockSeparator.cpp
index 6b52b1f..5fc5ba0 100644
--- a/clang/lib/Format/DefinitionBlockSeparator.cpp
+++ b/clang/lib/Format/DefinitionBlockSeparator.cpp
@@ -66,6 +66,10 @@
   };
   unsigned NewlineCount =
       (Style.SeparateDefinitionBlocks == FormatStyle::SDS_Always ? 1 : 0) + 1;
+
+  Style.MaxEmptyLinesToKeep =
+      std::max(Style.MaxEmptyLinesToKeep, NewlineCount - 1);
+
   WhitespaceManager Whitespaces(
       Env.getSourceManager(), Style,
       Style.LineEnding > FormatStyle::LE_CRLF

clang/lib/Format/Format.cpp

diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index e5533c3..574014f 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -4387,12 +4387,6 @@
     }
   }
 
-  if (Style.SeparateDefinitionBlocks != FormatStyle::SDS_Leave) {
-    Passes.emplace_back([&](const Environment &Env) {
-      return DefinitionBlockSeparator(Env, Expanded).process();
-    });
-  }
-
   if (Style.Language == FormatStyle::LK_ObjC &&
       !Style.ObjCPropertyAttributeOrder.empty()) {
     Passes.emplace_back([&](const Environment &Env) {
@@ -4411,6 +4405,12 @@
     return Formatter(Env, Expanded, Status).process();
   });
 
+  if (Style.SeparateDefinitionBlocks != FormatStyle::SDS_Leave) {
+    Passes.emplace_back([&](const Environment &Env) {
+      return DefinitionBlockSeparator(Env, Expanded).process();
+    });
+  }
+
   if (Style.isJavaScript() &&
       Style.InsertTrailingCommas == FormatStyle::TCS_Wrapped) {
     Passes.emplace_back([&](const Environment &Env) {

clang/unittests/Format/DefinitionBlockSeparatorTest.cpp

diff --git a/clang/unittests/Format/DefinitionBlockSeparatorTest.cpp b/clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
index 5e4c574..d18ab4e 100644
--- a/clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
+++ b/clang/unittests/Format/DefinitionBlockSeparatorTest.cpp
@@ -391,6 +391,35 @@
                Style, Prefix + Infix + Postfix);
 }
 
+TEST_F(DefinitionBlockSeparatorTest, AlwaysMaxEmptyLinesZeroAllman) {
+  FormatStyle Style = getLLVMStyle();
+  Style.BreakBeforeBraces = FormatStyle::BS_Allman;
+  Style.MaxEmptyLinesToKeep = 0;
+  Style.SeparateDefinitionBlocks = FormatStyle::SDS_Always;
+  Style.AllowShortFunctionsOnASingleLine = FormatStyle::ShortFunctionStyle();
+
+  verifyFormat("int my_function(int a)\n"
+               "\n"
+               "{\n"
+               "  return a;\n"
+               "}\n"
+               "int other_function(int a)\n"
+               "\n"
+               "{\n"
+               "  return a;\n"
+               "}",
+               Style,
+               "int my_function(int a)\n"
+               "{\n"
+               "  return a;\n"
+               "}\n"
+               "\n"
+               "int other_function(int a)\n"
+               "{\n"
+               "  return a;\n"
+               "}");
+}
+
 TEST_F(DefinitionBlockSeparatorTest, Never) {
   FormatStyle Style = getLLVMStyle();
   Style.SeparateDefinitionBlocks = FormatStyle::SDS_Never;