Put llvm and compiler-rt build directories in one place. (#30)

Adjust the llvm build script to put the llvm and compiler-rt build
directories outside the llvm-project source directory. A new build
directory 'build' at the toplevel of the revive directory is used
instead. LLVM is built into 'build/llvm' and compiler-rt into
'build/compiler-rt'.

Adjust .gitignore to ignore the contents of the build directory.

This is intended to keep the build artifacts separate from the upstream
sources and any changes made as part of the revive work.

.gitignore

@@ -13,3 +13,4 @@ artifacts
 tmp
 package-lock.json
 /*.html
+/build