Fix program name in "clang -help"

With out this patch,
"clang -help" prints "USAGE: clang-3 [options] <inputs>".
It should either print
USAGE: clang [options] <inputs>
or

USAGE: clang-3.7 [options] <inputs>
With this patch, on Linux, it prints
USAGE: clang-3.7 [options] <inputs>

On Windows, it prints
USAGE: clang.exe [options] <inputs>

llvm-svn: 231124
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 61aaa97..a0c9233 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -57,7 +57,7 @@
       CCGenDiagnostics(false), CCCGenericGCCName(""), CheckInputsExist(true),
       CCCUsePCH(true), SuppressMissingInputWarning(false) {
 
-  Name = llvm::sys::path::stem(ClangExecutable);
+  Name = llvm::sys::path::filename(ClangExecutable);
   Dir  = llvm::sys::path::parent_path(ClangExecutable);
 
   // Compute the path to the resource directory.