[ELF] - Rename PhdrEntry::First/Last to FirstSec/LastSec. NFC.

As was suggested in D34956 thread.

llvm-svn: 312712
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 3088a32..7292b70 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -770,7 +770,7 @@
 
   Out::ElfHeader->PtLoad = nullptr;
   Out::ProgramHeaders->PtLoad = nullptr;
-  FirstPTLoad->First = findFirstSection(FirstPTLoad);
+  FirstPTLoad->FirstSec = findFirstSection(FirstPTLoad);
 
   llvm::erase_if(Phdrs,
                  [](const PhdrEntry *E) { return E->p_type == PT_PHDR; });