cmake: really enable warnings with clang

Even though `PICKY_COMPILER=ON` is the default, warnings were not
enabled when using llvm/clang, because `CMAKE_COMPILER_IS_CLANG` was
always false (in my tests at least).

This is the single use of this variable in curl, and in a different
place we already use `CMAKE_C_COMPILER_ID MATCHES "Clang"`, which works
as expected, so change the condition to use that instead.

Also fix the warnings uncovered by the above:

- lib: add casts to silence clang warnings

- schannel: add casts to silence clang warnings in ALPN code

  Assuming the code is correct, solve the warnings with a cast.
  This particular build case isn't CI tested.

  There is a chance the warning is relevant for some platforms, perhaps
  Windows 32-bit ARM7.

Closes #9783

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