[DynamicLibrary] Fix build on musl libc

[loladiro]

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). Add an explicit cast to fix that problem.

[krytarowski]

What's the error message?

[loladiro]

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

[krytarowski]

This change looks fine. I'm unsure whether the proper style is to go for cast<void*>() or similar.

[dim]

LGTM.

[elram]

Has this been tested?
The modified EXPLICIT_SYMBOL is #undef at line 102 and does not apply to the EXPLICIT_SYMBOL(stdin/out/err) below.

llvm/lib/Support/Unix/DynamicLibrary.inc

void *DynamicLibrary::HandleSet::DLSym(void *Handle, const char *Symbol) {
   return nullptr;
 }
 
 #endif
 
 // 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
   // SearchForAddressOfSymbol to find them.  As such, we special case them here,
   // there is only a small handful of them.
 
 #ifdef __APPLE__
   {