Add nomerge function attribute to supress tail merge optimization in simplifyCFG

We want to add a way to avoid merging identical calls so as to keep the
separate debug-information for those calls. There is also an asan
usecase where having this attribute would be beneficial to avoid
alternative work-arounds.

Here is the link to the feature request:
https://bugs.llvm.org/show_bug.cgi?id=42783.

`nomerge` is different from `noline`. `noinline` prevents function from
inlining at callsites, but `nomerge` prevents multiple identical calls
from being merged into one.

This patch adds `nomerge` to disable the optimization in IR level. A
followup patch will be needed to let backend understands `nomerge` and
avoid tail merge at backend.

Reviewed By: asbirlea, rnk

Differential Revision: https://reviews.llvm.org/D78659

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -647,6 +647,8 @@ static uint64_t getAttrKindEncoding(Attribute::AttrKind Kind) {
     return bitc::ATTR_KIND_NO_INLINE;
   case Attribute::NoRecurse:
     return bitc::ATTR_KIND_NO_RECURSE;
+  case Attribute::NoMerge:
+    return bitc::ATTR_KIND_NO_MERGE;
   case Attribute::NonLazyBind:
     return bitc::ATTR_KIND_NON_LAZY_BIND;
   case Attribute::NonNull: