libcxx: Rename .hpp files in libcxx/test/support to .h

LLVM uses .h as its extension for header files.

Files renamed using:

    for f in libcxx/test/support/*.hpp; do git mv $f ${f%.hpp}.h; done

References to the files updated using:

    for f in $(git diff master | grep 'rename from' | cut -f 3 -d ' '); do
        a=$(basename $f);
        echo $a;
        rg -l $a libcxx | xargs sed -i '' "s/$a/${a%.hpp}.h/";
    done

HPP include guards updated manually using:

    for f in $(git diff master | grep 'rename from' | cut -f 3 -d ' '); do
      echo ${f%.hpp}.h ;
    done | xargs mvim

Differential Revision: https://reviews.llvm.org/D66104

llvm-svn: 369481
diff --git a/libcxx/test/support/filesystem_include.h b/libcxx/test/support/filesystem_include.h
new file mode 100644
index 0000000..f5c1d97
--- /dev/null
+++ b/libcxx/test/support/filesystem_include.h
@@ -0,0 +1,13 @@
+#ifndef TEST_SUPPORT_FILESYSTEM_INCLUDE_H
+#define TEST_SUPPORT_FILESYSTEM_INCLUDE_H
+
+#include <filesystem>
+#include "test_macros.h"
+
+#if defined(_LIBCPP_VERSION) && TEST_STD_VER < 17
+namespace fs = std::__fs::filesystem;
+#else
+namespace fs = std::filesystem;
+#endif
+
+#endif