[llvm-rc] Resolve the executable path if not present in Argv[0]

The llvm-rc tool tries to locate a suitable Clang executable to
use for preprocessing. For this purpose, it first checks within
the same directory as the llvm-rc tool, checking with a couple
different names, followed by checking all of $PATH for another
couple names.

On Windows, the InitLLVM() function always sets up Argv[0] with the
full path to the executable, while on Unix, Argv[0] is kept as is.

Therefore, call getMainExecutable to try to resolve the directory of
the executable before looking for colocated Clang executables.

This makes 282744a actually have
the desired effect.

Differential Revision: https://reviews.llvm.org/D157241

Author: mstorsjo

llvm/tools/llvm-rc/llvm-rc.cpp

@@ -123,7 +123,13 @@ std::string createTempFile(const Twine &Prefix, StringRef Suffix) {
 }
 
 ErrorOr<std::string> findClang(const char *Argv0, StringRef Triple) {
-  StringRef Parent = llvm::sys::path::parent_path(Argv0);
+  // This just needs to be some symbol in the binary.
+  void *P = (void*) (intptr_t) findClang;
+  std::string MainExecPath = llvm::sys::fs::getMainExecutable(Argv0, P);
+  if (MainExecPath.empty())
+    MainExecPath = Argv0;
+
+  StringRef Parent = llvm::sys::path::parent_path(MainExecPath);
   ErrorOr<std::string> Path = std::error_code();
   std::string TargetClang = (Triple + "-clang").str();
   std::string VersionedClang = ("clang-" + Twine(LLVM_VERSION_MAJOR)).str();