[AVR] Update the rust-lang/llvm-project submodule to include AVR fixes recently merged

This commit updates rustc's LLVM submodule to include new AVR-specific
fixes recently merged on the Rust LLVM 'rustc/10.0-2020-05-05' branch.

All of these cherry-picked commits exist in upstream LLVM and were
cherry-picked into Rust's LLVM fork in commit 6c040dd86ed.

  |- 6c040dd86ed Merge pull request rust-lang#66 from dylanmckay/avr-pick-upstream-llvm-fixes
    |- 12dfdd3aed7 [AVR] Rewrite the function calling convention.
    |- 118ac53f12b [AVR] Don't adjust for instruction size
    |- bc27c282e13 [AVR] Fix miscompilation of zext + add
    |- cfbe205a7e8 [AVR] Remove faulty stack pushing behavior
    |- 143e1469e96 [AVR] Fix stack size in functions with a frame pointer
    |- 6b2445d841e [LLVM][AVR] Support for R_AVR_6 fixup
    |- 93ee4da19cf [AVR]  Fix I/O instructions on XMEGA
    |- 962c2415ffb [AVR] Do not place functions in .progmem.data
    |- 65b8b170aef [AVR] Do not use divmod calls for bigger integers
    |- 93a3b595d1c [AVR] Generalize the previous interrupt bugfix to signal
    |- handlers too
    |- cc4286349b4 [AVR] Respect the 'interrupt' function attribute
    |- 954d0a92205 [AVR] Fix reads of uninitialized variables from constructor of AVRSubtarget
    |- 1c0ddae73c9 [AVR] Fix read of uninitialized variable AVRSubtarget:::ELFArch
    |- 0ed0823fe60 [AVR] Fix incorrect register state for LDRdPtr
    |- 96075fc433d [AVR] Don't adjust addresses by 2 for absolute values
    |- 6dfc55ba53b [AVR] Use correct register class for mul instructions

These changes include both correctness fixes and LLVM assertion error
fixes. Once all of these commits have been cherry-picked, all of the
LLVM plumbing for rust-lang/master to compile the AVR blink program will
be in place. Once this commit is merged, only PR rust-lang#73270 will
be blocking successful compilation and emission of the AVR LED blink program.

Author: dylanmckay