[clang] NFC: Remove forced type merging in multiversion function checks.

Checking of multiversion function declarations performed by various functions
in clang/lib/Sema/SemaDecl.cpp previously forced the valus of a passed in
'MergeTypeWithPrevious' reference argument in several scenarios. This was
unnecessary and possibly incorrect in the one case that the value
was forced to 'true' (though seemingly unobservably so).

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D121958