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.
i think we should drop Invalid from the signature completely if we are going for this.
I've sent out D82532 to get rid of the libclang dependency on this, after it lands this should be possible to clear.