instream: pass fileName by reference

... to eliminate unnecessary string copies

Suggested by Coverity:
```
Error: COPY_INSTEAD_OF_MOVE:
src/cslinker.cc:236:29: copy_constructor_call: "fnCwe" is passed-by-value as parameter to "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const &)", when it could be moved instead.
src/cslinker.cc:236:29: remediation: Use "std::move(""fnCwe"")" instead of "fnCwe".

Error: COPY_INSTEAD_OF_MOVE:
src/cslinker.cc:249:29: copy_constructor_call: "fnImp" is passed-by-value as parameter to "std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const &)", when it could be moved instead.
src/cslinker.cc:249:29: remediation: Use "std::move(""fnImp"")" instead of "fnImp".
```

Related: #216

Author: kdudka

src/lib/instream.cc

@@ -19,8 +19,8 @@
 
 #include "instream.hh"
 
-InStream::InStream(std::string fileName, const bool silent):
-    fileName_(std::move(fileName)),
+InStream::InStream(const std::string &fileName, const bool silent):
+    fileName_(fileName),
     silent_(silent),
     str_((fileName_ == "-")
                 ? std::cin

src/lib/instream.hh

@@ -38,7 +38,7 @@ struct InFileException {
 
 class InStream {
     public:
-        InStream(std::string fileName, bool silent = false);
+        InStream(const std::string &fileName, bool silent = false);
         InStream(std::istringstream &str, bool silent = false);
         ~InStream() = default;
 

src/lib/parser-json-cov.cc

@@ -89,4 +89,3 @@ bool CovTreeDecoder::readNode(Defect *def)
 
     return true;
 }
-