[clang-format] Indent tokens after hash only if it starts a line

[owenpan]

Summary

Fixes https://github.com/llvm/llvm-project/issues/56602.

[curdeius]

LGTM.

clang/lib/Format/ContinuationIndenter.cpp

void ContinuationIndenter::addTokenOnCurrentLine(LineState &State, bool DryRun,
   }
 
   unsigned Spaces = Current.SpacesRequiredBefore + ExtraSpaces;
 
   // Indent preprocessor directives after the hash if required.
   int PPColumnCorrection = 0;
   if (Style.IndentPPDirectives == FormatStyle::PPDIS_AfterHash &&
       Previous.is(tok::hash) && State.FirstIndent > 0 &&
+      &Previous == State.Line->First &&
       (State.Line->Type == LT_PreprocessorDirective ||
        State.Line->Type == LT_ImportStatement)) {
     Spaces += State.FirstIndent;
 
     // For preprocessor indent with tabs, State.Column will be 1 because of the
     // hash. This causes second-level indents onward to have an extra space
     // after the tabs. We avoid this misalignment by subtracting 1 from the
     // column value passed to replaceWhitespace().

clang/unittests/Format/FormatTest.cpp

verifyFormat("#ifdef _WIN32\n"
                "#define MACRO                          \\\n"
                "  some_very_long_func_aaaaaaaaaa();\n"
                "#endif\n"
                "#else\n"
                "#define A 1\n"
                "#endif",
                Style);
   Style.IndentPPDirectives = FormatStyle::PPDIS_AfterHash;
+  verifyFormat("#if 1\n"
+               "#  define __STR(x) #x\n"
+               "#endif",
+               Style);
   verifyFormat("#ifdef _WIN32\n"
                "#  define A 0\n"
                "#  ifdef VAR2\n"
                "#    define B 1\n"
                "#    include <someheader.h>\n"
                "#    define MACRO                      \\\n"
                "      some_very_long_func_aaaaaaaaaa();\n"
                "#  endif\n"