Comment parsing: resolve more named character references

This reimplements r173850 with a better approach:
(1) use a TableGen-generated matcher instead of doing a linear search;
(2) avoid allocations for new strings by converting code points to string
    iterals with TableGen.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173931 91177308-0d34-0410-b5e6-96231b3b80d8