Drop -Xcpp, adopt DMD's -P instead

CHANGELOG.md

@@ -3,7 +3,7 @@
 #### Big news
 - Frontend, druntime and Phobos are at version [2.103.1](https://dlang.org/changelog/2.103.0.html), incl. new command-line option `-verror-supplements`. (#4345)
 - New commandline option `-femit-local-var-lifetime` that enables variable lifetime (scope) annotation to LLVM IR codegen. Lifetime annotation enables stack memory reuse for local variables with non-overlapping scope. (#4395)
-- C files are now automatically preprocessed using the external C compiler (configurable via `-gcc` or the `CC` environment variable, and `-Xcc` for extra flags). Extra preprocessor flags (e.g., include dirs and manual defines) can be added via new command-line option `-Xcpp`. (#4417)
+- C files are now automatically preprocessed using the external C compiler (configurable via `-gcc` or the `CC` environment variable, and `-Xcc` for extra flags). Extra preprocessor flags (e.g., include dirs and manual defines) can be added via new command-line option `-P`. (#4417)
   - Windows: Requires an MSVC environment set up for the target (`cl.exe` on `PATH`, `INCLUDE` environment variable). If `clang-cl.exe` is on `PATH`, it is preferred over Microsoft's `cl.exe` by default (e.g., to avoid printing the C source file path to stderr during preprocessing).
 
 #### Platform support

driver/cl_options.cpp

@@ -404,7 +404,7 @@ cl::list<std::string> ccSwitches(
     "Xcc", cl::value_desc("ccflag"), cl::cat(linkingCategory),
     cl::desc("Pass <ccflag> to GCC/Clang for linking/preprocessing"));
 
-cl::list<std::string> cppSwitches("Xcpp", cl::value_desc("cppflag"),
+cl::list<std::string> cppSwitches("P", cl::value_desc("cppflag"), cl::Prefix,
                                   cl::desc("Pass <cppflag> to C preprocessor"));
 
 cl::opt<std::string> moduleDeps(

driver/ldmd.cpp

@@ -709,11 +709,8 @@ void translateArgs(const llvm::SmallVectorImpl<const char *> &ldmdArgs,
         exit(EXIT_SUCCESS);
       }
       /* -L
-       */
+       * -P
-      else if (p[1] == 'P') {
+       * -defaultlib
-        ldcArgs.push_back(concat("-Xcpp=", p + 2 + (p[2] == '=' ? 1 : 0)));
-      }
-      /* -defaultlib
        * -debuglib
        * -deps
        * -main