MC: Tweak variable assignment diagnostics, and make reassignment of non-absolute

variables and symbols invalid.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84232 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCAsmStreamer.cpp

@@ -125,6 +125,7 @@ void MCAsmStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
   OS << '\n';
 
   // FIXME: Lift context changes into super class.
+  // FIXME: Set associated section.
   Symbol->setValue(Value);
 }