Add support for generating missing GNU build notes to the LLVM assembler.
This makes clang accept and pass through -Wa,--generate-missing-build-notes=[yes/no]. If =yes is passed (the default is no), then the assembler will check whether a .gnu.build.attributes section is present. If not, it will generate one for all relevant code sections.
This is the equivalent to what the GNU assembler gas is already doing in maybe_generate_build_notes() in gas/write.c: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gas/write.c;h=0b43063bbaab76d21fe3bac5fb4643bfff3c72e2;hb=HEAD#l1954