Fix dangling-else bugs in FAIL_FAST_IF and EMIT_USER_WARNING macros (#41504)

* Fix dangling-else bugs in FAIL_FAST_IF and EMIT_USER_WARNING macros  

Both macros were bare if-statements without do/while(0) guards, causing
the dangling else problem when used as a single statement under an if.

* Fix dangling-else bugs in FAIL_FAST_IF and EMIT_USER_WARNING macros

* harden EMIT_USER_WARNING macro on Windows against dangling-else

* Fix dangling-else bugs in FAIL_FAST_IF and EMIT_USER_WARNING macros