[ms inline asm] Add support for label names with '$' chars

[myatsina]

Summary

In MS inline asm syntax a label with '$' char produces an error, while in AT&T it does not.
In AT&T inline asm syntax Clang escapes the '$' char and replaces it with "$$". Adopted same approach for MS syntax.

[• ehsan]

LGTM.

lib/Sema/SemaStmtAsm.cpp

if (Label->isMSAsmLabel()) {
     Label->markUsed(Context);
   } else {
     // Otherwise, insert it, but only resolve it if we have seen the label itself.
     std::string InternalName;
     llvm::raw_string_ostream OS(InternalName);
     // Create an internal name for the label.  The name should not be a valid mangled
     // name, and should be unique.  We use a dot to make the name an invalid mangled
     // name.
-    OS << "__MSASMLABEL_." << MSAsmLabelNameCounter++ << "__" << ExternalLabelName;
+    OS << "__MSASMLABEL_." << MSAsmLabelNameCounter++ << "__";
+    for (auto it = ExternalLabelName.begin(); it != ExternalLabelName.end();
+         ++it) {
+      OS << *it;
+      if (*it == '$') {
+        // We escape '$' in asm strings by replacing it with "$$"
+        OS << '$';
+      }
+    }
     Label->setMSAsmLabel(OS.str());
   }
   if (AlwaysCreate) {
     // The label might have been created implicitly from a previously encountered
     // goto statement.  So, for both newly created and looked up labels, we mark
     // them as resolved.
     Label->setMSAsmLabelResolved();
   }
   // Adjust their location for being able to generate accurate diagnostics.
   Label->setLocation(Location);
 
   return Label;
 }

test/CodeGen/ms-inline-asm.c

void label4() {
   __asm {
     label:
     mov eax, [label]
   }
   // CHECK-LABEL: define void @label4
   // CHECK: call void asm sideeffect inteldialect "{{.*}}__MSASMLABEL_.4__label:\0A\09mov eax, {{.*}}__MSASMLABEL_.4__label", "~{eax},~{dirflag},~{fpsr},~{flags}"()
 }
 
+void label5() {
+  __asm {
+    jmp dollar_label$
+    dollar_label$:
+  }
+  // CHECK-LABEL: define void @label5
+  // CHECK: call void asm sideeffect inteldialect "jmp {{.*}}__MSASMLABEL_.5__dollar_label$$\0A\09{{.*}}__MSASMLABEL_.5__dollar_label$$:", "~{dirflag},~{fpsr},~{flags}"()
+}
+
 typedef union _LARGE_INTEGER {
   struct {
     unsigned int LowPart;
     unsigned int  HighPart;
   };
   struct {
     unsigned int LowPart;
     unsigned int  HighPart;