MCExpr: Add FindAssociatedSection, which attempts to mirror the 'as' semantics

that associate sections with expressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130517 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCSymbol.h

@@ -56,6 +56,7 @@ namespace llvm {
     mutable unsigned IsUsed : 1;
 
   private:  // MCContext creates and uniques these.
+    friend class MCExpr;
     friend class MCContext;
     MCSymbol(StringRef name, bool isTemporary)
       : Name(name), Section(0), Value(0),