[clang-format] Propagate LeadingEmptyLinesAffected when joining lines

Before this commit, when `LineJoiner` joins a line with affected
leading whitespace, it would drop the knowledge of this
entirely. However, when the `AffectedRangeManager` is computing the
affected lines, the leading empty whitespace lines are potentially
considered for non-first tokens in the `AnnotatedLine`. This causes a
discrepancy in behavior when an `AnnotatedLine` is put together from
joining multiple lines versus when it is not.

We change `LineJoiner::join` to follow `AffectedRangeManager`'s logic,
considering the leading whitespace when determining `Affected` for a
token.

https://github.com/llvm/llvm-project/blob/a63f57262898588b576d66e5fd79c0aa64b35f2d/clang/lib/Format/AffectedRangeManager.cpp#L111-L130

Fixes #138942.

Author: tJener

clang/lib/Format/UnwrappedLineFormatter.cpp

@@ -986,8 +986,10 @@ class LineJoiner {
   void join(AnnotatedLine &A, const AnnotatedLine &B) {
     assert(!A.Last->Next);
     assert(!B.First->Previous);
-    if (B.Affected)
+    if (B.Affected || B.LeadingEmptyLinesAffected) {
+      assert(B.Affected || A.Last->Children.empty());
       A.Affected = true;
+    }
     A.Last->Next = B.First;
     B.First->Previous = A.Last;
     B.First->CanBreakBefore = true;

clang/unittests/Format/FormatTestSelective.cpp

@@ -41,6 +41,11 @@ TEST_F(FormatTestSelective, RemovesTrailingWhitespaceOfFormattedLine) {
   EXPECT_EQ("int a;", format("int a;         ", 0, 0));
   EXPECT_EQ("int a;\n", format("int a;  \n   \n   \n ", 0, 0));
   EXPECT_EQ("int a;\nint b;    ", format("int a;  \nint b;    ", 0, 0));
+
+  EXPECT_EQ("void f() {}", format("void f() {\n"
+                                  "  \n"
+                                  "}",
+                                  11, 0));
 }
 
 TEST_F(FormatTestSelective, FormatsCorrectRegionForLeadingWhitespace) {