Test most StaticAssert macros in C++ extensions

Test most StaticAssert macros in C++ extensions

Most of the StaticAssert macros already worked in C++ with Clang and
GCC:(the only compilers we're currently testing C++ extension support
for).  This adds a regression test for them in our test C++ extension,
so we can safely change their implementation without accidentally
breaking C++.

The only macros that StaticAssert macros that don't work yet are the
StaticAssertVariableIsOfType and StaticAssertVariableIsOfTypeMacro.
These will be added in a follow-on commit.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg@mail.gmail.com