Rename VAStart -> ImageBase. NFC.

Config members are named after corresponding command line options.
This patch renames VAStart ImageBase so that they are in line with
--image-base.

Differential Revision: http://reviews.llvm.org/D22277

llvm-svn: 275298
diff --git a/lld/ELF/Target.cpp b/lld/ELF/Target.cpp
index c27af42..8e520fb 100644
--- a/lld/ELF/Target.cpp
+++ b/lld/ELF/Target.cpp
@@ -250,7 +250,9 @@
   return 0;
 }
 
-uint64_t TargetInfo::getVAStart() const { return Config->Pic ? 0 : VAStart; }
+uint64_t TargetInfo::getImageBase() const {
+  return Config->Pic ? 0 : ImageBase;
+}
 
 bool TargetInfo::usesOnlyLowPageBits(uint32_t Type) const { return false; }
 
@@ -989,7 +991,7 @@
   //
   // And because the lowest non-zero 256M boundary is 0x10000000, PPC64 linkers
   // use 0x10000000 as the starting address.
-  VAStart = 0x10000000;
+  ImageBase = 0x10000000;
 }
 
 static uint64_t PPC64TocOffset = 0x8000;