LibC: Drop complex.cpp and move its definitions to complex.h

libc++ disallows including LibC's complex.h in C++ mode. This means that
a C++ file cannot expect LibC's complex.h to be included, and thus
cannot use c-prefixed complex number functions. As a result,
complex.cpp is broken when libc++ has a higher include priority
than LibC.

A check for __cplusplus has been added to complex.h to warn users of
toolchains that don't use libc++.

Userland/Libraries/LibC/CMakeLists.txt

@@ -1,7 +1,6 @@
 set(LIBC_SOURCES
     arpa/inet.cpp
     assert.cpp
-    complex.cpp
     ctype.cpp
     cxxabi.cpp
     dirent.cpp