[PM] Edit comments in PassManager.h.

Summary:
This covers most of PassManager.h, up to the introduction of inner/outer
analysis proxies.

If there's a theme to these changes, it's simplifying the language.  For
example:

  * PreservedAnalyses is a "set of analyses", not an "abstract set".
    "Abstract" doesn't have any particular meaning here.

  * "Build types for the concept types" becomes "define the concept types".

  * Instead of "data structures optimized for pointer-like types using
    the alignment-provided low bits", say "data structures that use the
    low bits of pointers."

  * "Clear the map pointing into the results list" becomes
    "Delete the map entries that point into the results list."

This patch also fixes a few places where we referred to "function" and
"module" pass/analysis managers, instead of the more abstract "IRUnitT"
PM/AMs we have now.

Subscribers: mehdi_amini

Differential Revision: https://reviews.llvm.org/D27367

llvm-svn: 291040