Update submodules and switch to llvm 9 branch (#102)

This change was mostly generated by changing the upstream llvm
branch to 9.x and running:

  $ git submodule update --remote

As well as switching the llvm 9 this change also bring in the
following wasi-libc changes:

  5933c20 fix macos filename, use https
  7c39519 CI: upgrade to llvm 9.0.0
  9ca5187 remove no-self-update workaround for windows azure
  9580a25 deprecate azure pipelines CI, build libc on GH Actions
  2c2fc9a Don't call `free` on paths which are about to call `_Exit`. (#161)
  c6f2c05 gen-headers: Generate assertions of layout from witx (#149)
  37c663f Correct minor typo in c_headers.rs (#166)
  12f5832 Convert more wasi-libc code to `//`-style comments. (#153)
  ec86d4d Improvements to wasi-headers tool (#160)
  1fad338 Fix environment variable init to exit successfully. (#159)
  a280fea Move math source files. (#151)
  dd010be Avoid using cast expressions in WASI API constants. (#148)

Fixes: #101

Author: sbc100

.gitmodules

@@ -1,7 +1,7 @@
 [submodule "src/llvm-project"]
 	path = src/llvm-project
 	url = https://github.com/llvm/llvm-project
-	branch = release/8.x
+	branch = release/9.x
 [submodule "src/wasi-libc"]
 	path = src/wasi-libc
 	url = https://github.com/CraneStation/wasi-libc

README.md

@@ -15,12 +15,13 @@ wasi-libc tree.
 The libc portion of this SDK is the
 [wasi-libc](https://github.com/CraneStation/wasi-libc).
 
-Upstream Clang and LLVM 8.0 can compile for WASI out of the box, and WebAssembly
-support is included in them by default. So, all that's done here is to provide
-builds configured to set the default target and sysroot for convenience.
+Upstream Clang and LLVM (from 8.0 onwards) can compile for WASI out of the box,
+and WebAssembly support is included in them by default. So, all that's done here
+is to provide builds configured to set the default target and sysroot for
+convenience.
 
-One could also use a standard Clang 8.0, build a sysroot from the sources
-mentioned above, and compile with
+One could also use a standard Clang installation, build a sysroot from the
+sources mentioned above, and compile with
 "--target=wasm32-wasi --sysroot=/path/to/sysroot".
 
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