Change linkage computation so it doesn't depend on FunctionDecl::isExternC or...

Change linkage computation so it doesn't depend on FunctionDecl::isExternC or VarDecl::isExternC, and instead queries what it actually cares about: whether the given declaration is inside an extern "C" context.  Fundamentally, figuring out whether a function/variable uses C linkage requires knowing the linkage, and the logic in FunctionDecl::isExternC and VarDecl::isExternC was getting it wrong.  Given that, fix FunctionDecl::isExternC and VarDecl::isExternC to use much simpler implementations that depend on the fixed linkage computation.

Fixes a regression to test/SemaCXX/linkage.cpp caused by a new warning exposing the fact that the internal state was wrong.

llvm-svn: 148207