[Driver] Use the parent_path of the clang executable as the default InstalledDir

This is what most people want anyways. Clang -cc1's main() will override
this but for other tools this is the most sensible default and avoids
some work.

llvm-svn: 250164
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 025e2de2..dc95ed2 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -65,6 +65,7 @@
 
   Name = llvm::sys::path::filename(ClangExecutable);
   Dir = llvm::sys::path::parent_path(ClangExecutable);
+  InstalledDir = Dir; // Provide a sensible default installed dir.
 
   // Compute the path to the resource directory.
   StringRef ClangResourceDir(CLANG_RESOURCE_DIR);