[MC]  Add assembler support for .cg_profile.

Object FIle Representation
At codegen time this is emitted into the ELF file a pair of symbol indices and a weight. In assembly it looks like:

.cg_profile a, b, 32
.cg_profile freq, a, 11
.cg_profile freq, b, 20

When writing an ELF file these are put into a SHT_LLVM_CALL_GRAPH_PROFILE (0x6fff4c02) section as (uint32_t, uint32_t, uint64_t) tuples as (from symbol index, to symbol index, weight).

Differential Revision: https://reviews.llvm.org/D44965

llvm-svn: 333823
diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index 89d418d..491fec3 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -661,6 +661,10 @@
     getContext().reportError(Loc, "Chained unwind areas can't have handlers!");
 }
 
+void MCStreamer::emitCGProfileEntry(const MCSymbolRefExpr *From,
+                                    const MCSymbolRefExpr *To, uint64_t Count) {
+}
+
 static MCSection *getWinCFISection(MCContext &Context, unsigned *NextWinCFIID,
                                    MCSection *MainCFISec,
                                    const MCSection *TextSec) {