In commit 8d61294c3e (PCH: Mark CMake PCH source files as -x
<lang>-header, 2020-09-04, v3.18.3~14^2) we removed the explicit `-x
objective-c++` flag. This broke cases with custom source extensions.
Restore the explicit `-x objective-c[++]` flag and put it before the
`<FLAGS>` placeholder. The latter will contain the proper `-x
objective-c[++]-header` value and will override the `-x objective-c[++]`
value set before.
Fixes: #21234
