Eliminate DefaultImageName from the Driver constructor

All callers were passing in "a.out" or garbage so a sensible default works fine
here as a cleanup.

This also brings about the possibility of adapting the value based on the
driver's compatibility mode in future.

The setting can still be changed via Driver::DefaultImageName as needed.

llvm-svn: 208926

clang/lib/Frontend/CreateInvocationFromCommandLine.cpp

@@ -47,7 +47,7 @@ clang::createInvocationFromCommandLine(ArrayRef<const char *> ArgList,
 
   // FIXME: We shouldn't have to pass in the path info.
   driver::Driver TheDriver("clang", llvm::sys::getDefaultTargetTriple(),
-                           "a.out", *Diags);
+                           *Diags);
 
   // Don't check that inputs exist, they may have been remapped.
   TheDriver.setCheckInputsExist(false);