[gn build] Port 18d4ba5

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/AsmParser/BUILD.gn

@@ -6,6 +6,7 @@ static_library("AsmParser") {
     "//llvm/lib/Support",
   ]
   sources = [
+    "AsmParserContext.cpp",
     "LLLexer.cpp",
     "LLParser.cpp",
     "Parser.cpp",