Add support for llvm-mc as an assembly linter (#4446)

test/handler/test_llvm_mc_handler.vader

@@ -0,0 +1,28 @@
+Before:
+  runtime ale_linters/asm/llvm_mc.vim
+
+After:
+  call ale#linter#Reset()
+
+Execute(The asm llvm-mc handler should parse lines correctly):
+
+  AssertEqual
+  \ [
+  \    {
+  \      'lnum': 10,
+  \      'col' : 15,
+  \      'text': "invalid operand for instruction",
+  \      'type': 'E',
+  \    },
+  \    {
+  \      'lnum': 11,
+  \      'col' : 2,
+  \      'text': "invalid instruction mnemonic 'lpaq'",
+  \      'type': 'E',
+  \    },
+  \ ],
+  \ ale_linters#asm#llvm_mc#Handle(357, [
+  \   "xorq %rbp, %rbp",
+  \   "{standard_input}:10:15: error: invalid operand for instruction",
+  \   "{standard input}:11:2: error: invalid instruction mnemonic 'lpaq'",
+  \ ])

test/linter/test_llvm_mc.vader

@@ -0,0 +1,19 @@
+Before:
+  call ale#assert#SetUpLinterTest('asm', 'llvm_mc')
+  call ale#test#SetFilename('test.cpp')
+  let b:command_tail = ' --assemble'
+  \   . ' --filetype=asm'
+  \   . ' -o ' . (has('win32') ? 'nul': '/dev/null')
+  \   . ' '
+
+After:
+  unlet! b:command_tail
+
+  call ale#assert#TearDownLinterTest()
+
+Execute(The executable should be configurable):
+  AssertLinter 'llvm-mc', ale#Escape('llvm-mc') . b:command_tail
+
+  let b:ale_asm_llvm_mc_executable = 'foobar'
+
+  AssertLinter 'foobar', ale#Escape('foobar') . b:command_tail