Type Type::isRealFloatingType() that vectors are not floating-point

Type Type::isRealFloatingType() that vectors are not floating-point
types, updating callers of both isFloatingType() and
isRealFloatingType() accordingly. Caught at least one issue where we
allowed one to declare a vector of vectors (!), along with cleaning up
the standard-conversion logic for C++.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@106595 91177308-0d34-0410-b5e6-96231b3b80d8