Fix an assertion failure in FormatASTNodeDiagnosticArgument.

Summary:
The test being added in this patch used to cause an assertion failure:

/build/./bin/clang -cc1 -internal-isystem /build/lib/clang/5.0.0/include -nostdsysteminc -verify -fsyntax-only -std=c++11 -Wshadow-all /src/tools/clang/test/SemaCXX/warn-shadow.cpp
--
Exit Code: 134

Command Output (stderr):
--
clang: /src/tools/clang/lib/AST/ASTDiagnostic.cpp:424: void clang::FormatASTNodeDiagnosticArgument(DiagnosticsEngine::ArgumentKind, intptr_t, llvm::StringRef, llvm::StringRef, ArrayRef<DiagnosticsEngine::ArgumentValue>, SmallVectorImpl<char> &, void *, ArrayRef<intptr_t>): Assertion `isa<NamedDecl>(DC) && "Expected a NamedDecl"' failed.
#0 0x0000000001c7a1b4 PrintStackTraceSignalHandler(void*) (/build/./bin/clang+0x1c7a1b4)
chapuni#1 0x0000000001c7a4e6 SignalHandler(int) (/build/./bin/clang+0x1c7a4e6)
chapuni#2 0x00007f30880078d0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0xf8d0)
chapuni#3 0x00007f3087054067 gsignal (/lib/x86_64-linux-gnu/libc.so.6+0x35067)
#4 0x00007f3087055448 abort (/lib/x86_64-linux-gnu/libc.so.6+0x36448)
#5 0x00007f308704d266 (/lib/x86_64-linux-gnu/libc.so.6+0x2e266)
#6 0x00007f308704d312 (/lib/x86_64-linux-gnu/libc.so.6+0x2e312)
#7 0x00000000035b7f22 clang::FormatASTNodeDiagnosticArgument(clang::DiagnosticsEngine::ArgumentKind, long, llvm::StringRef, llvm::StringRef, llvm::ArrayRef<std::pair<clang::DiagnosticsEngine::ArgumentKind, long> >, llvm::SmallVectorImpl<char>&, void*, llvm::ArrayRef<long>) (/build/
./bin/clang+0x35b7f22)
#8 0x0000000001ddbae4 clang::Diagnostic::FormatDiagnostic(char const*, char const*, llvm::SmallVectorImpl<char>&) const (/build/./bin/clang+0x1ddbae4)
#9 0x0000000001ddb323 clang::Diagnostic::FormatDiagnostic(char const*, char const*, llvm::SmallVectorImpl<char>&) const (/build/./bin/clang+0x1ddb323)
#10 0x00000000022878a4 clang::TextDiagnosticBuffer::HandleDiagnostic(clang::DiagnosticsEngine::Level, clang::Diagnostic const&) (/build/./bin/clang+0x22878a4)
#11 0x0000000001ddf387 clang::DiagnosticIDs::ProcessDiag(clang::DiagnosticsEngine&) const (/build/./bin/clang+0x1ddf387)
#12 0x0000000001dd9dea clang::DiagnosticsEngine::EmitCurrentDiagnostic(bool) (/build/./bin/clang+0x1dd9dea)
#13 0x0000000002cad00c clang::Sema::EmitCurrentDiagnostic(unsigned int) (/build/./bin/clang+0x2cad00c)
#14 0x0000000002d91cd2 clang::Sema::CheckShadow(clang::NamedDecl*, clang::NamedDecl*, clang::LookupResult const&) (/build/./bin/clang+0x2d91cd2)

Stack dump:
0.      Program arguments: /build/./bin/clang -cc1 -internal-isystem /build/lib/clang/5.0.0/include -nostdsysteminc -verify -fsyntax-only -std=c++11 -Wshadow-all /src/tools/clang/test/SemaCXX/warn-shadow.cpp
1.      /src/tools/clang/test/SemaCXX/warn-shadow.cpp:214:23: current parser token ';'
2.      /src/tools/clang/test/SemaCXX/warn-shadow.cpp:213:26: parsing function body 'handleLinkageSpec'
3.      /src/tools/clang/test/SemaCXX/warn-shadow.cpp:213:26: in compound statement ('{}')
/build/tools/clang/test/SemaCXX/Output/warn-shadow.cpp.script: line 1: 15595 Aborted                 (core dumped) /build/./bin/clang -cc1 -internal-isystem /build/lib/clang/5.0.0/include -nostdsysteminc -verify -fsyntax-only -std=c++11 -Wshadow-all /src/tools/clang/test/SemaCXX/warn-shadow.cpp

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: krytarowski, cfe-commits

Differential Revision: https://reviews.llvm.org/D33207

clang/lib/Sema/SemaDecl.cpp

@@ -6935,7 +6935,7 @@ void Sema::CheckShadow(NamedDecl *D, NamedDecl *ShadowedDecl,
         }
     }
 
-  DeclContext *OldDC = ShadowedDecl->getDeclContext();
+  DeclContext *OldDC = ShadowedDecl->getDeclContext()->getRedeclContext();
 
   unsigned WarningDiag = diag::warn_decl_shadow;
   SourceLocation CaptureLoc;

clang/test/SemaCXX/warn-shadow.cpp

@@ -206,3 +206,10 @@ void avoidWarningWhenRedefining(int b) { // expected-note {{previous definition
 }
 
 }
+
+extern "C" {
+typedef int externC; // expected-note {{previous declaration is here}}
+}
+void handleLinkageSpec() {
+  typedef void externC; // expected-warning {{declaration shadows a typedef in the global namespace}}
+}