[clang-tidy] Fix a false positive on modernize-use-nullptr check.

Summary:
The false positive happens on two neighbour CXXDefaultArgExpr AST nodes.
like below:

```
CXXFunctionalCastExpr 0x85c9670 <col:7, col:23> 'struct ZZ' functional cast to struct ZZ <ConstructorConversion>
 `-CXXConstructExpr 0x85c9518 <col:7, col:23> 'struct ZZ' 'void (uint64, const uint64 *)'
   |-CallExpr 0x85a0a90 <col:10, col:22> 'uint64':'unsigned long long'
   | |-ImplicitCastExpr 0x85a0a78 <col:10> 'uint64 (*)(uint64)' <FunctionToPointerDecay>
   | | `-DeclRefExpr 0x85a09f0 <col:10> 'uint64 (uint64)' lvalue Function 0x85a06a0 'Hash' 'uint64 (uint64)'
   | `-CXXDefaultArgExpr 0x85a0ac8 <<invalid sloc>> 'uint64':'unsigned long long'
   `-CXXDefaultArgExpr 0x85c94f8 <<invalid sloc>> 'const uint64 *'
```

For each particular CXXDefaultArgExpr node, we need to reset
FirstSubExpr, otherwise FirstSubExpr will refer to an incorrect expr.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D30412

llvm-svn: 296479