clang-tidy modernize-loop-convert: preserve type of alias declaration (bug 28341)

Summary:
Previoly, the added test failed with the fillowing fixit:

     char v[5];

-    for(size_t i = 0; i < 5; ++i)
+    for(char value : v)
     {
-        unsigned char value = v[i];
         if (value > 127)

i.e. the variable 'value' changes from unsigned char to signed char. And
thus the following 'if' does not work anymore.

With this commit, the fixit is changed to:
     char v[5];

-    for(size_t i = 0; i < 5; ++i)
+    for(unsigned char value : v)
     {
-        unsigned char value = v[i];
         if (value > 127)

Reviewers: alexfh, klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D22069

llvm-svn: 276111