llvm: option to build without assertions.

Closes Homebrew/homebrew#16449.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/llvm.rb

@@ -35,6 +35,7 @@ class Llvm < Formula
   option 'shared', 'Build LLVM as a shared library'
   option 'all-targets', 'Build all target backends'
   option 'rtti', 'Build with C++ RTTI'
+  option 'disable-assertions', 'Speeds up LLVM, but provides less debug information'
 
   def install
     if build.universal? and build.include? 'shared'
@@ -72,6 +73,8 @@ class Llvm < Formula
     end
     args << "--enable-shared" if build.include? 'shared'
 
+    args << "--disable-assertions" if build.include? 'disable-assertions'
+
     system "./configure", *args
     system "make install"