cfi-icall: Allow the jump table to be optionally made non-canonical.

cfi-icall: Allow the jump table to be optionally made non-canonical.

The default behavior of Clang's indirect function call checker will replace
the address of each CFI-checked function in the output file's symbol table
with the address of a jump table entry which will pass CFI checks. We refer
to this as making the jump table `canonical`. This property allows code that
was not compiled with ``-fsanitize=cfi-icall`` to take a CFI-valid address
of a function, but it comes with a couple of caveats that are especially
relevant for users of cross-DSO CFI:

- There is a performance and code size overhead associated with each
  exported function, because each such function must have an associated
  jump table entry, which must be emitted even in the common case where the
  function is never address-taken anywhere in the program, and must be used
  even for direct calls between DSOs, in addition to the PLT overhead.

- There is no good way to take a CFI-valid address of a function written in
  assembly or a language not supported by Clang. The reason is that the code
  generator would need to insert a jump table in order to form a CFI-valid
  address for assembly functions, but there is no way in general for the
  code generator to determine the language of the function. This may be
  possible with LTO in the intra-DSO case, but in the cross-DSO case the only
  information available is the function declaration. One possible solution
  is to add a C wrapper for each assembly function, but these wrappers can
  present a significant maintenance burden for heavy users of assembly in
  addition to adding runtime overhead.

For these reasons, we provide the option of making the jump table non-canonical
with the flag ``-fno-sanitize-cfi-canonical-jump-tables``. When the jump
table is made non-canonical, symbol table entries point directly to the
function body. Any instances of a function's address being taken in C will
be replaced with a jump table address.

This scheme does have its own caveats, however. It does end up breaking
function address equality more aggressively than the default behavior,
especially in cross-DSO mode which normally preserves function address
equality entirely.

Furthermore, it is occasionally necessary for code not compiled with
``-fsanitize=cfi-icall`` to take a function address that is valid
for CFI. For example, this is necessary when a function's address
is taken by assembly code and then called by CFI-checking C code. The
``__attribute__((cfi_jump_table_canonical))`` attribute may be used to make
the jump table entry of a specific function canonical so that the external
code will end up taking a address for the function that will pass CFI checks.

Fixes PR41972.

Differential Revision: https://reviews.llvm.org/D65629

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368495 91177308-0d34-0410-b5e6-96231b3b80d8