Print vasm next to LLVM IR.

Summary: When 'llvm' trace level is set to 5 and above we print corresponding vasm instructions next to LLVM IR.

Reviewed By: @swtaarrs

Differential Revision: D1701264

Signature: t1:1701264:1416872125:08e8b99b4d9f5012db02e308b98f4808d231cc70
Generate new_delete linker command programmatically

Summary: ASAN doesn't like redefining new/delete, probably because it wants
distinct symbols for the purpose of matching up new/delete vs malloc/free.  The
workaround here is to generate the new_delete linker file programmatically and
make it conditional on FOLLY_SANITIZE_ADDRESS.

Reviewed By: @markw65

Differential Revision: D1711991