Use the simpler version of llvm::sys::fs::exists.

In all these cases it looks like the intention was to handle error in a similar
way to the file not existing.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@217614 91177308-0d34-0410-b5e6-96231b3b80d8
clang-cl: Add support for the /o option for object files, executables, and preprocessor output

Summary:
cl.exe recognizes /o as a deprecated and undocumented option similar to
/Fe.  This patch adds support for this option to clang-cl for /Fe, /Fo
and /Fi.  It also ensures that the last option among /o and /F* wins,
if both specified.

This is required at least for building autoconf based software, since
autoconf uses -o to specify the executable output.

This fixes http://llvm.org/PR20894.

Test Plan: The patch includes automated tests.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5308

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@217615 91177308-0d34-0410-b5e6-96231b3b80d8