[NFC] Rename variable to workaround old gcc bug

[aeubanks]

Summary

gcc 5.1 is still supported according to
https://releases.llvm.org/10.0.0/docs/GettingStarted.html

We're hitting the following bug due to a variable created in the loop header being the same as a variable used in the loop header:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54430

[hctim]

LGTM - thanks.

compiler-rt/lib/hwasan/hwasan_report.cpp

 // we don't have a binary with symbols, we can't grab the size of the global
 // from the debug info - but we might be able to retrieve it from the
 // descriptor. Returns zero if the lookup failed.
 static uptr GetGlobalSizeFromDescriptor(uptr ptr) {
   // Find the ELF object that this global resides in.
   Dl_info info;
   dladdr(reinterpret_cast<void *>(ptr), &info);
   auto *ehdr = reinterpret_cast<const ElfW(Ehdr) *>(info.dli_fbase);
-  auto *phdr = reinterpret_cast<const ElfW(Phdr) *>(
+  auto *phdr_begin = reinterpret_cast<const ElfW(Phdr) *>(
       reinterpret_cast<const u8 *>(ehdr) + ehdr->e_phoff);
 
   // Get the load bias. This is normally the same as the dli_fbase address on
   // position-independent code, but can be different on non-PIE executables,
   // binaries using LLD's partitioning feature, or binaries compiled with a
   // linker script.
   ElfW(Addr) load_bias = 0;
   for (const auto &phdr :
-       ArrayRef<const ElfW(Phdr)>(phdr, phdr + ehdr->e_phnum)) {
+       ArrayRef<const ElfW(Phdr)>(phdr_begin, phdr_begin + ehdr->e_phnum)) {
     if (phdr.p_type != PT_LOAD || phdr.p_offset != 0)
       continue;
     load_bias = reinterpret_cast<ElfW(Addr)>(ehdr) - phdr.p_vaddr;
     break;
   }
 
   // Walk all globals in this ELF object, looking for the one we're interested
   // in. Once we find it, we can stop iterating and return the size of the
   // global we're interested in.
   for (const hwasan_global &global :
-       HwasanGlobalsFor(load_bias, phdr, ehdr->e_phnum))
+       HwasanGlobalsFor(load_bias, phdr_begin, ehdr->e_phnum))
     if (global.addr() <= ptr && ptr < global.addr() + global.size())
       return global.size();
 
   return 0;
 }
 
 void PrintAddressDescription(
     uptr tagged_addr, uptr access_size,