Improve obvious-most-derived-type devirtualization:

Improve obvious-most-derived-type devirtualization:

  * if the base is produced by a series of derived-to-base conversions, check
    the expression inside them when looking for an expression with a known
    dynamic type
  * step past MaterializeTemporaryExprs when checking for a known dynamic type
  * when checking for a known dynamic type, treat all class prvalues as having
    a known dynamic type after skipping all relevant rvalue subobject
    adjustments
  * treat callees formed by pointer-to-member access for a non-reference member
    type like callees formed by member access.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@285954 91177308-0d34-0410-b5e6-96231b3b80d8