VS 2013 Miscompiles

[Aaron Ballman]

While updating clang to VS 2013, I've seen numerous miscompiles on x86
that did not appear in VS 2012 related to the Redeclarable class.  It
seems that VS 2012 would properly calculate the offset to the base
class, while VS 2013 would not.  Specifically, VS 2012 would generate
the following code:

  RecordDecl *getPreviousDecl() {
025FF040  push        ebp
025FF041  mov         ebp,esp
025FF043  push        ecx
025FF044  mov         dword ptr [this],0CCCCCCCCh
025FF04B  mov         dword ptr [this],ecx
    return cast_or_null<RecordDecl>(TagDecl::getPreviousDecl());
025FF04E  mov         ecx,dword ptr [this]
025FF051  add         ecx,34h
025FF054  call
clang::Redeclarable<clang::TagDecl>::getPreviousDecl (025FEF40h)
025FF059  push        eax
025FF05A  call
llvm::cast_or_null<clang::RecordDecl,clang::TagDecl> (025A9850h)
025FF05F  add         esp,4
  }

VS 2013, however, is missing the add ecx, 34h instruction:

  RecordDecl *getPreviousDecl() {
02500010  push        ebp
02500011  mov         ebp,esp
02500013  push        ecx
02500014  mov         dword ptr [this],0CCCCCCCCh
0250001B  mov         dword ptr [this],ecx
    return cast_or_null<RecordDecl>(TagDecl::getPreviousDecl());
0250001E  mov         ecx,dword ptr [this]
02500021  call
clang::Redeclarable<clang::TagDecl>::getPreviousDecl (024FFF10h)
02500026  push        eax
02500027  call
llvm::cast_or_null<clang::RecordDecl,clang::TagDecl> (024AA9B0h)
0250002C  add         esp,4

This would cause crashes in getPreviousDecl.  Unfortunately, this also
affects other calls on Redeclarable such as getMostRecentDecl.

This patch "addresses" the miscompile such that it now compiles
properly for VS 2013 as well as VS 2012.  Instead of calling through a
scoped lookup on a method coming from Redeclarable, it statically
casts the this pointer and calls the method.  This seems to force
Visual Studio to generate the proper code.

~Aaron
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