[clang][tools] Silence const cast warning when building with Clang ToT

This fixes:
```
[4960/7446] Building C object tools\clang\tools\c-index-test\CMakeFiles\c-index-test.dir\c-index-test.c.obj
C:\git\llvm-project\clang\tools\c-index-test\c-index-test.c(49,19): warning: cast from 'const char *' to 'char *' drops const qualifier [-Wcast-qual]
   49 |     return((char*)path);
      |                   ^
C:\git\llvm-project\clang\tools\c-index-test\c-index-test.c(239,18): warning: cast from 'const char *' to 'char *' drops const qualifier [-Wcast-qual]
  239 |     free((char *)unsaved_files[i].Filename);
      |                  ^
C:\git\llvm-project\clang\tools\c-index-test\c-index-test.c(240,18): warning: cast from 'const char *' to 'char *' drops const qualifier [-Wcast-qual]
  240 |     free((char *)unsaved_files[i].Contents);
      |                  ^
```