Add IR support, ELF section and user documentation for partitioning feature.

The partitioning feature was proposed here:
http://lists.llvm.org/pipermail/llvm-dev/2019-February/130583.html

This is mostly just documentation. The feature itself will be contributed
in subsequent patches.

Differential Revision: https://reviews.llvm.org/D60242

llvm-svn: 361923
diff --git a/llvm/lib/AsmParser/LLLexer.cpp b/llvm/lib/AsmParser/LLLexer.cpp
index c0b9cd1..dc8ff7f 100644
--- a/llvm/lib/AsmParser/LLLexer.cpp
+++ b/llvm/lib/AsmParser/LLLexer.cpp
@@ -570,6 +570,7 @@
   KEYWORD(align);
   KEYWORD(addrspace);
   KEYWORD(section);
+  KEYWORD(partition);
   KEYWORD(alias);
   KEYWORD(ifunc);
   KEYWORD(module);