Support: Remove needless allocation when getMainExecutable() calls readlink()

[thakis]

Summary

We built a StringRef from a string literal which we then converted to a
std::string to call c_str(). Just use a pointer to the string literal
instead of a StringRef.

No behavior change.

[rnk]

lgtm

Huh, code seems to date from 2012: rL164676.

llvm/lib/Support/Unix/Path.inc

#if defined(__APPLE__)
   if (_NSGetExecutablePath(exe_path, &size) == 0) {
     char link_path[MAXPATHLEN];
     if (realpath(exe_path, link_path))
       return link_path;
   }
 #elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||   \
     defined(__minix) || defined(__DragonFly__) ||                              \
     defined(__FreeBSD_kernel__) || defined(_AIX)
-  StringRef curproc("/proc/curproc/file");
+  const char *curproc = "/proc/curproc/file";
   char exe_path[PATH_MAX];
   // /proc is not mounted by default under FreeBSD, but gives more accurate
   // information than argv[0] when it is.
   if (sys::fs::exists(curproc)) {
-    ssize_t len = readlink(curproc.str().c_str(), exe_path, sizeof(exe_path));
+    ssize_t len = readlink(curproc, exe_path, sizeof(exe_path));
     if (len > 0) {
       // Null terminate the string for realpath. readlink never null
       // terminates its output.
       len = std::min(len, ssize_t(sizeof(exe_path) - 1));
       exe_path[len] = '\0';
       return exe_path;
     }
   }
   // If we don't have procfs mounted, fall back to argv[0]
   if (getprogpath(exe_path, argv0) != NULL)
     return exe_path;
 #elif defined(__linux__) || defined(__CYGWIN__)
   char exe_path[MAXPATHLEN];
-  StringRef aPath("/proc/self/exe");
+  const char *aPath = "/proc/self/exe";
   if (sys::fs::exists(aPath)) {
     // /proc is not always mounted under Linux (chroot for example).
-    ssize_t len = readlink(aPath.str().c_str(), exe_path, sizeof(exe_path));
+    ssize_t len = readlink(aPath, exe_path, sizeof(exe_path));
     if (len < 0)
       return "";
 
     // Null terminate the string for realpath. readlink never null
     // terminates its output.
     len = std::min(len, ssize_t(sizeof(exe_path) - 1));
     exe_path[len] = '\0';