[clang] Try fixing implicit-module-header-maps.cpp

This is still crashing on AIX and Solaris. It looks like maybe issues
due to trying to delete the current working directory. cd to the source
directory beforehand to try and work around that.

Author: boomanaiden154

clang/test/Modules/implicit-module-header-maps.cpp

@@ -1,4 +1,5 @@
 // UNSUPPORTED: system-windows
+// RUN: cd %S
 // RUN: rm -rf %t
 // RUN: mkdir %t
 // RUN: split-file %s %t
@@ -8,6 +9,7 @@
 //
 // RUN: %clang -Rmodule-build -fmodules -fimplicit-modules -fimplicit-module-maps -fmodule-map-file=module.modulemap -fsyntax-only -I hmap -fmodules-cache-path=%t test.cpp
 //
+// RUN: cd %S
 // RUN: rm -rf %t
 // RUN: mkdir %t
 // RUN: split-file %s %t