Fix llvm-config behavior when LLVM_TOOLS_INSTALL_DIR has a depth greater than one

[serge-sans-paille]

Summary

llvm-config makes the assumption that LLVM_TOOLS_INSTALL_DIR is a single directory. Choosing a (relative) path with a depth greater than one leads to invalid prefix computation.

[serge-sans-paille]

@sylvestre.ledru up :-)

[sylvestre.ledru]

I guess it isn't possible to write a test for this?

tools/llvm-config/llvm-config.cpp

int main(int argc, char **argv) {
   bool HasAnyOption = false;
 
   // llvm-config is designed to support being run both from a development tree
   // and from an installed path. We try and auto-detect which case we are in so
   // that we can report the correct information when run from a development
   // tree.
   bool IsInDevelopmentTree;
   enum { CMakeStyle, CMakeBuildModeStyle } DevelopmentTreeLayout;
-  llvm::SmallString<256> CurrentPath(GetExecutablePath(argv[0]));
   std::string CurrentExecPrefix;
   std::string ActiveObjRoot;
 
   // If CMAKE_CFG_INTDIR is given, honor it as build mode.
   char const *build_mode = LLVM_BUILDMODE;
 #if defined(CMAKE_CFG_INTDIR)
   if (!(CMAKE_CFG_INTDIR[0] == '.' && CMAKE_CFG_INTDIR[1] == '\0'))
     build_mode = CMAKE_CFG_INTDIR;
 #endif
 
-  // Create an absolute path, and pop up one directory (we expect to be inside a
-  // bin dir).
+  // Create an absolute path, and pop up as much directory as in
+  // LLVM_TOOLS_INSTALL_DIR
+  {
+    llvm::SmallString<256> CurrentPath(GetExecutablePath(argv[0]));
     sys::fs::make_absolute(CurrentPath);
-  CurrentExecPrefix =
-      sys::path::parent_path(sys::path::parent_path(CurrentPath)).str();
+    for (auto iter = sys::path::begin(LLVM_TOOLS_INSTALL_DIR),
+              end = sys::path::end(LLVM_TOOLS_INSTALL_DIR);
+         iter != end; ++iter) {
+      CurrentPath = sys::path::parent_path(CurrentPath).str();
+    }
+    CurrentExecPrefix = sys::path::parent_path(CurrentPath).str();
+  }
 
   // Check to see if we are inside a development tree by comparing to possible
   // locations (prefix style or CMake style).
   if (sys::fs::equivalent(CurrentExecPrefix, LLVM_OBJ_ROOT)) {
     IsInDevelopmentTree = true;
     DevelopmentTreeLayout = CMakeStyle;
     ActiveObjRoot = LLVM_OBJ_ROOT;
   } else if (sys::fs::equivalent(CurrentExecPrefix,