headerscheck ccache support

Currently, headerscheck and cpluspluscheck are very slow, and they
defeat use of ccache.  This fixes that, and now they are much faster.

The problem was that the test files are created in a randomly-named
directory (`mktemp -d /tmp/$me.XXXXXX`), and this directory is
mentioned on the compiler command line, which is part of the cache
key.

The solution is to create the test files in the build directory.  For
example, for src/include/storage/ipc.h, we generate

    tmp_headerscheck_c/src_include_storage_ipc_h.c (or .cpp)

Now ccache works.  (And it's also a bit easier to debug everything
with this naming.)

(The subdirectory is used to keep the cleanup trap simple.)

The observed speedup on Cirrus CI for headerscheck plus cpluspluscheck
is from about 1min 20s to only 20s.  In local use, the speedups are
similar.

Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/b49e74d4-3cf9-4d1c-9dce-09f75e55d026%40eisentraut.org

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