Add -f[no-]strict-return flag that can be used to avoid undefined behaviour

Add -f[no-]strict-return flag that can be used to avoid undefined behaviour
in non-void functions that fall off at the end without returning a value when
compiling C++.

Clang uses the new compiler flag to determine when it should treat control flow
paths that fall off the end of a non-void function as unreachable. If
-fno-strict-return is on, the code generator emits the ureachable and trap
IR only when the function returns either a record type with a non-trivial
destructor or another non-trivially copyable type.

The primary goal of this flag is to avoid treating falling off the end of a
non-void function as undefined behaviour. The burden of undefined behaviour
is placed on the caller instead: if the caller ignores the returned value then
the undefined behaviour is avoided. This kind of behaviour is useful in
several cases, e.g. when compiling C code in C++ mode.

rdar://13102603

Differential Revision: https://reviews.llvm.org/D27163

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@290960 91177308-0d34-0410-b5e6-96231b3b80d8