Fix crash on `user defined literals`

Summary:
Given an UserDefinedLiteral `1.2_w`:
Problem: Lexer generates one Token for the literal, but ClangAST
references two source locations
Fix: Ignore the operator and interpret it as the underlying literal.
e.g.: `1.2_w` token generates syntax node IntegerLiteral(1.2_w)

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82157