Update LLVM build system for 249b40b558955afe5ac2b549edcf2d7f859c8cc9

This enables the use of modules for a significant build performance gain
when building with clang as host compiler or when using

Update LLVM build system for 249b40b558955afe5ac2b549edcf2d7f859c8cc9

This enables the use of modules for a significant build performance gain
when building with clang as host compiler or when using HAVE_LLVM=yes.
Switch libc++ to using the copy from the mono-repo.

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Build and install llvm-symbolizer for MKLLVM=yes

llvm-symbolizer is an alternative for GNU addr2line(1), heavily used by the
LLVM sanitizers.

Do not install it as tools as it is not necessary as of

Build and install llvm-symbolizer for MKLLVM=yes

llvm-symbolizer is an alternative for GNU addr2line(1), heavily used by the
LLVM sanitizers.

Do not install it as tools as it is not necessary as of today in that
stage.

show more ...

Fix harmless syntax error.

Update LLVM to 10.0.0git (01f3a59fb3e2542fce74c768718f594d0debd0da)