llvm: add NDEBUG when assertion mode is off

[haampie]

llvm-config --cxxflags unfortunately does not return -DNDEBUG when LLVM is
built with it, but Julia is required to have this define when including LLVM
header files. I didn't check if llvm-config --cxxflags has changed at some
point in LLVM (I think so because Julia switched from explicit -DNDEBUG to
llvm-config --cxxflags in v0.2.0 ages ago [1]), but the llvm-config --assertion-mode
flag was introduced a little after Julia dropped explicit -DNDEBUG [2].

Found this when compiling Julia with PGO enabled, which simply fails to link
because of virtual functions LLVM guards behind ifdefs.

[1] d2f6a78
[2] llvm-mirror/llvm@0427957

[haampie]

Hm, static analysis doesn't like this. It follows a path where Log2_64(0) is called which returns 63 - 64 -> 4294967295 and is then used as a shift. Is it a false positive or an actual issue?

[vtjnash]

SGTM. I believe LLVM removed this from the default flags sometime between versions 6 and 8 (Julia v1.3 and v1.4) without us noticing. Until then, we had src/julia_assert.h working to ensure this was working as intended.

vtjnash@arctic4:/data/vtjnash$ ./julia13/usr/tools/llvm-config --cflags
-I/data/vtjnash/julia13/usr/include   -fPIC -Werror=date-time -Wall -W -Wno-unused-parameter -Wwrite-strings -Wno-missing-field-initializers -pedantic -Wno-long-long -Wundef -Wno-comment -ffunction-sections -fdata-sections -O3 -DNDEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

vtjnash@arctic4:/data/vtjnash$ ./julia13/usr/tools/llvm-config --version
6.0.1

vtjnash@arctic4:/data/vtjnash$ ./julia14/usr/tools/llvm-config --cflags
-I/data/vtjnash/julia14/usr/include  -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS

vtjnash@arctic4:/data/vtjnash$ ./julia14/usr/tools/llvm-config --version
8.0.1jl

[vtjnash]

You might need to add -UNDEBUG to the analyze-gc step flags to counteract this. Otherwise, it can end up losing invariant information that has been annotated, for the sake of static analysis, such as this assert:
https://llvm.org/doxygen/Alignment_8h_source.html#l00077

[vtjnash]

bump?