Revert "[MC] Remove assert to work around BOLT"

This reverts commit c72cb27.
The patch was not reviewed, and this assertion guards against incorrect
values for symbols if the linker decides to modify the layout as it
happens on some platforms. getSymbolOffset() is used for code size
estimation, among others.

Reverting for now, until we get proper code review and undersatnding of
the implications.

Author: dcci

llvm/lib/MC/MCAssembler.cpp

@@ -538,6 +538,7 @@ bool MCAssembler::getSymbolOffset(const MCSymbol &S, uint64_t &Val) const {
 }
 
 uint64_t MCAssembler::getSymbolOffset(const MCSymbol &S) const {
+  assert(HasLayout);
   uint64_t Val;
   getSymbolOffsetImpl(*this, S, true, Val);
   return Val;