[MC] Speed up AttemptToFoldSymbolOffsetDifference in the absence of MCAsmLayout

The `FA < FB` check added by https://reviews.llvm.org/D153096 is slow.
Compute an informal layout order to speed up computation when
`AttemptToFoldSymbolOffsetDifference` is repeatedly called for the same
section.

Commit 9500a5d0 ("[MC] Make UseAssemblerInfoForParsing mostly true")
exposed this performance pitfall, which was mitigated by
`setUseAssemblerInfoForParsing(false)` workarounds (e.g. commit
245491a9). The workaround can be removed
now.