[AST] Fix the CXXFoldExpr source range when parentheses range is invalid.

The CXXFoldExpr's range is invalid if the cxxfoldexpr is formed via the
Concept's TypeContraints (because the parentheses are not written in the
source code). We fallback to use the range from the pattern.

Differential Revision: https://reviews.llvm.org/D85645