clang-cl: Don't look up absolute paths in %LIB%.

Before this patch, passing a non-existent absolute path to clang-cl would cause
stat'ing of impossible paths. For example, `clang-cl -c d:\adsfasdf.txt` would
cause a stat of
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\LIBd:\asdfadsf.cc

llvm-svn: 235787
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 07a5e42..7e52b1f 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -989,7 +989,8 @@
   if (llvm::sys::fs::exists(Twine(Path)))
     return true;
 
-  if (D.IsCLMode() && llvm::sys::Process::FindInEnvPath("LIB", Value))
+  if (D.IsCLMode() && !llvm::sys::path::is_absolute(Twine(Path)) &&
+      llvm::sys::Process::FindInEnvPath("LIB", Value))
     return true;
 
   D.Diag(clang::diag::err_drv_no_such_file) << Path;