[DynamicLibrary] Fix build on musl libc

Summary:
On musl libc, stdin/out/err are defined as `FILE* const` globals,
and their address is not implicitly convertible to void *,
or at least gcc 6 doesn't allow it, giving errors like:

```
error: cannot initialize return object of type 'void *' with an rvalue of type 'FILE *const *' (aka '_IO_FILE *const *')
    EXPLICIT_SYMBOL(stderr);
    ^~~~~~~~~~~~~~~~~~~~~~~
```

Add an explicit cast to fix that problem.

Reviewers: marsupial, krytarowski, dim
Reviewed By: dim
Differential Revision: https://reviews.llvm.org/D39297

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316672 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Unix/DynamicLibrary.inc

@@ -71,7 +71,7 @@ void *DynamicLibrary::HandleSet::DLSym(void *Handle, const char *Symbol) {
 // Must declare the symbols in the global namespace.
 static void *DoSearch(const char* SymbolName) {
 #define EXPLICIT_SYMBOL(SYM) \
-   extern void *SYM; if (!strcmp(SymbolName, #SYM)) return &SYM
+   extern void *SYM; if (!strcmp(SymbolName, #SYM)) return (void*)&SYM
 
   // If this is darwin, it has some funky issues, try to solve them here.  Some
   // important symbols are marked 'private external' which doesn't allow