[clang-tidy] Fix case of local variables in modernize-use-nodiscard checker

Summary:
Correct the case of the local variables..

Rational:
I want to be able to run clang-tidy on new clang-tidy checker code prior to creating a review (to demonstrate we should dog food our own tools during development, not my suggestion but @Eugene.Zelenko)

To this end I am running the following in a script, prior to make a change.

```
tidy:
    @for source in $$(git status -suno | grep ".cpp$$" | cut -c4-) ;\
    do \
        clang-tidy -quiet  $$source -- $(TIDY_FLAGS);\
    done

```

I then want to go through the checkers and see which checkers most closely match the review style of the reviewers

```
---
Checks:          '
-clang-diagnostic-*,
readability-identifier-naming,
llvm-header-guard
'
WarningsAsErrors: ''
HeaderFilterRegex: ''
AnalyzeTemporaryDtors: false
FormatStyle:     LLVM
CheckOptions:
  - key:             readability-identifier-naming.IgnoreFailedSplit
    value:           '0'
  - key:             readability-identifier-naming.VariableCase
    value:           'CamelCase'
  - key:             readability-identifier-naming.LocalVariableCase
    value:           'CamelCase'
...

```

Unfortunately in doing so, I have identified that my previous review {D55433} it violates what looks like to be the convention of local variables being in CamelCase.

Sending this small review in the hope it can be corrected.

Patch by MyDeveloperDay.

Reviewers: JonasToth, Eugene.Zelenko

Reviewed By: JonasToth

Subscribers: xazax.hun, Eugene.Zelenko

Differential Revision: https://reviews.llvm.org/D56536

llvm-svn: 350814