[clang-tidy] Fix FP with readability-redundant-string-init for default arguments

Summary:
Clang-tidy is reporting a warning of redundant string initialisation
on a string parameter initialized with empty string.

See bug: 27087

The reported example is:
```
#include <string>
void fn(std::string a = "");
```

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18829

llvm-svn: 265671

1 files changed, 2 insertions, 1 deletions

diff --git a/clang-tools-extra/clang-tidy/readability/RedundantStringInitCheck.cpp b/clang-tools-extra/clang-tidy/readability/RedundantStringInitCheck.cpp
index d49c55e..2997e5d 100644
--- a/clang-tools-extra/clang-tidy/readability/RedundantStringInitCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/RedundantStringInitCheck.cpp
@@ -61,7 +61,8 @@ void RedundantStringInitCheck::registerMatchers(MatchFinder *Finder) {
                         hasInitializer(
                             expr(anyOf(EmptyStringCtorExpr,
                                        EmptyStringCtorExprWithTemporaries))
-                            .bind("expr"))))
+                            .bind("expr"))),
+                unless(parmVarDecl()))
           .bind("decl"),
       this);
 }