[clang-repl] Implement continuation for preprocessor directives. (#107552)

Author: vgvassilev

clang/test/Interpreter/multiline.cpp

@@ -1,6 +1,8 @@
 // REQUIRES: host-supports-jit
 // UNSUPPORTED: system-aix
-// RUN: cat %s | clang-repl | FileCheck %s
+// RUN: cat %s | clang-repl -Xcc -Xclang -Xcc -verify | FileCheck %s
+
+// expected-no-diagnostics
 
 extern "C" int printf(const char*,...);
 int i = \
@@ -17,8 +19,7 @@ void f(int x) \
 f(i);
 // CHECK: x=12
 
-// FIXME: Support preprocessor directives.
-// #if 0 \
-//   #error "Can't be!" \
-// #endif
+#if 0                   \
+  #error "Can't be!"    \
+#endif
 

clang/tools/clang-repl/ClangRepl.cpp

@@ -232,8 +232,10 @@ int main(int argc, const char **argv) {
       llvm::StringRef L = *Line;
       L = L.trim();
       if (L.ends_with("\\")) {
-        // FIXME: Support #ifdef X \ ...
         Input += L.drop_back(1);
+        // If it is a preprocessor directive, new lines matter.
+        if (L.starts_with('#'))
+          Input += "\n";
         LE.setPrompt("clang-repl...   ");
         continue;
       }