Revert "Re-commit r192758 - MC: quote tricky symbol names in asm output"

This caused the clang-native-mingw32-win7 buildbot to break.

The assembler was complaining about the following lines that were showing up
in the asm for CrashRecoveryContext.cpp:

  movl  $"__ZL16ExceptionHandlerP19_EXCEPTION_POINTERS@4", 4(%eax)
  calll "_AddVectoredExceptionHandler@8"
  .def   "__ZL16ExceptionHandlerP19_EXCEPTION_POINTERS@4";
  "__ZL16ExceptionHandlerP19_EXCEPTION_POINTERS@4":
  calll "_RemoveVectoredExceptionHandler@4"

Reverting for now.

llvm-svn: 192940
diff --git a/llvm/lib/MC/MCSymbol.cpp b/llvm/lib/MC/MCSymbol.cpp
index f386c3b..b973c57 100644
--- a/llvm/lib/MC/MCSymbol.cpp
+++ b/llvm/lib/MC/MCSymbol.cpp
@@ -18,10 +18,12 @@
   reinterpret_cast<const MCSection *>(1);
 
 static bool isAcceptableChar(char C) {
-  return (C >= 'a' && C <= 'z') ||
-         (C >= 'A' && C <= 'Z') ||
-         (C >= '0' && C <= '9') ||
-         C == '_' || C == '$' || C == '.';
+  if ((C < 'a' || C > 'z') &&
+      (C < 'A' || C > 'Z') &&
+      (C < '0' || C > '9') &&
+      C != '_' && C != '$' && C != '.' && C != '@')
+    return false;
+  return true;
 }
 
 /// NameNeedsQuoting - Return true if the identifier \p Str needs quotes to be