Remove bad test.

We currently don't canonicalize paths in the preprocessed files.
But we do when writing to PCH.
This causes a discrepancy on Windows with the test below.
This test fails even on unix if you change the test to use
`%S//preprocess.h`.

I am led to conclude that the invariant that this test was intending to
test has not been upheld for a while (and may never have been).

llvm-svn: 243602

clang/test/PCH/preprocess.c

@@ -1,8 +0,0 @@
-// Check that -E mode is invariant when using an implicit PCH.
-
-// RUN: %clang_cc1 -include %S/preprocess.h -E -o %t.orig %s
-// RUN: %clang_cc1 -emit-pch -o %t %S/preprocess.h
-// RUN: %clang_cc1 -include-pch %t -E -o %t.from_pch %s
-// RUN: diff %t.orig %t.from_pch
-
-a_typedef a_value;

clang/test/PCH/preprocess.h

@@ -1,7 +0,0 @@
-// Helper header for preprocess.c PCH test
-#ifndef PREPROCESS_H
-#define PREPROCESS_H
-
-typedef int a_typedef;
-
-#endif // PREPROCESS_H