[SourceManager] don't check invalid param of getLocalSLocEntry()

Forked from D80681.

getLocalSLocEntry() has an unused parameter used to satisfy an interface
of libclang (see getInclusions() in
clang/tools/libclang/CIndexInclusionStack.cpp).  It's pointless for
callers to construct/pass/check this inout parameter that can never
signify that a FileID is invalid.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D82498