[clang-scan-deps] Fix "unterminated conditional directive" bug

`clang-scan-deps` threw "unterminated conditional directive" error
falsely on the following example:

```

 #if defined(__TEST_DUMMY2)
  #pragma GCC warning                                                          \
      "Hello!"
 #else
  #pragma GCC error                                                            \
      "World!"
 #endif // defined(__TEST_DUMMY2)

```

The issue comes from PR llvm#143950, where the flag `LastNonWhitespace`
does not correctly represent the state in the example above.  The PR
aimed to support that a line-continuation can be followed by
whitespaces.  This fix uses a more straightforward but less efficient
way to do the same thing---it looks back for a line-continuation and
skips any number of whitespaces before that.

rdar://153742186

Author: ziqingluo-90

clang/lib/Lex/DependencyDirectivesScanner.cpp

@@ -419,7 +419,7 @@ static bool isQuoteCppDigitSeparator(const char *const Start,
 }
 
 void Scanner::skipLine(const char *&First, const char *const End) {
-  char LastNonWhitespace = ' ';
+  const char * const OldFirst = First;
   for (;;) {
     assert(First <= End);
     if (First == End)
@@ -453,8 +453,6 @@ void Scanner::skipLine(const char *&First, const char *const End) {
       // Iterate over comments correctly.
       if (*First != '/' || End - First < 2) {
         LastTokenPtr = First;
-        if (!isWhitespace(*First))
-          LastNonWhitespace = *First;
         ++First;
         continue;
       }
@@ -467,8 +465,6 @@ void Scanner::skipLine(const char *&First, const char *const End) {
 
       if (First[1] != '*') {
         LastTokenPtr = First;
-        if (!isWhitespace(*First))
-          LastNonWhitespace = *First;
         ++First;
         continue;
       }
@@ -480,9 +476,26 @@ void Scanner::skipLine(const char *&First, const char *const End) {
       return;
 
     // Skip over the newline.
-    skipNewline(First, End);
-
-    if (LastNonWhitespace != '\\')
+    auto Len = skipNewline(First, End);
+    // Since P2223R2 allows the line-continuation slash \ to be followed by
+    // additional whitespace, we need to check that here if `First`
+    // follows a `\\` and whitespaces.
+
+    // `LookBack` points to the character before the newline:
+    const char *LookBack = First - Len;
+    bool LineContinuationFound = false;
+
+    // Move `LookBack` backwards to find line-continuation and whitespaces:
+    while (LookBack >= OldFirst) { // bound `LookBack` by `OldFirst`:
+      if (isWhitespace(*LookBack)) {
+        --LookBack; // whitespace before '\\' is ok
+        continue;
+      }
+      if (*LookBack == '\\')
+        LineContinuationFound = true;
+      break; // not a whitespace, end loop
+    }
+    if (!LineContinuationFound)
       break;
   }
 }

clang/unittests/Lex/DependencyDirectivesScannerTest.cpp

@@ -880,6 +880,37 @@ TEST(MinimizeSourceToDependencyDirectivesTest,
   EXPECT_EQ(pp_eof, Directives[22].Kind);
 }
 
+TEST(MinimizeSourceToDependencyDirectivesTest,
+     TestFixedBugThatReportUnterminatedDirectiveFalsely) {
+  SmallVector<char, 512> Out;
+  SmallVector<dependency_directives_scan::Token, 16> Tokens;
+  SmallVector<Directive, 16> Directives;
+
+  StringRef Input = "#ifndef __TEST \n"
+                    "#define __TEST \n"
+                    "#if defined(__TEST_DUMMY) \n"
+                    "#if defined(__TEST_DUMMY2) \n"
+                    "#pragma GCC warning        \\ \n"
+                    "\"hello!\"\n"
+                    "#else\n"
+                    "#pragma GCC error          \\ \n"
+                    "\"world!\" \n"
+                    "#endif // defined(__TEST_DUMMY2) \n"
+                    "#endif  // defined(__TEST_DUMMY) \n"
+                    "#endif // #ifndef __TEST \n";
+  ASSERT_FALSE( // False on no error:
+      minimizeSourceToDependencyDirectives(Input, Out, Tokens, Directives));
+  ASSERT_TRUE(Directives.size() == 8);
+  EXPECT_EQ(pp_ifndef, Directives[0].Kind);
+  EXPECT_EQ(pp_define, Directives[1].Kind);
+  EXPECT_EQ(pp_if, Directives[2].Kind);
+  EXPECT_EQ(pp_if, Directives[3].Kind);
+  EXPECT_EQ(pp_endif, Directives[4].Kind);
+  EXPECT_EQ(pp_endif, Directives[5].Kind);
+  EXPECT_EQ(pp_endif, Directives[6].Kind);
+  EXPECT_EQ(pp_eof, Directives[7].Kind);
+}
+
 TEST(MinimizeSourceToDependencyDirectivesTest, PoundWarningAndError) {
   SmallVector<char, 128> Out;