WString: mark move ctor as noexcept (#7610)

ref.
- https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-move-noexcept
- https://rules.sonarsource.com/cpp/RSPEC-5018?search=noexecept
- https://clang.llvm.org/extra/clang-tidy/checks/performance-noexcept-move-constructor.html

> Move constructors of all the types used with STL containers, for
example, need to be declared noexcept. Otherwise STL will choose copy
constructors instead. The same is valid for move assignment operations.

cores/esp8266/WString.cpp

@@ -46,12 +46,12 @@ String::String(const __FlashStringHelper *pstr) {
 }
 
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-String::String(String &&rval) {
+String::String(String &&rval) noexcept {
     init();
     move(rval);
 }
 
-String::String(StringSumHelper &&rval) {
+String::String(StringSumHelper &&rval) noexcept {
     init();
     move(rval);
 }
@@ -224,7 +224,7 @@ String & String::copy(const __FlashStringHelper *pstr, unsigned int length) {
 }
 
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-void String::move(String &rhs) {
+void String::move(String &rhs) noexcept {
     if (buffer()) {
         if (capacity() >= rhs.len()) {
             memmove_P(wbuffer(), rhs.buffer(), rhs.length() + 1);
@@ -267,13 +267,13 @@ String & String::operator =(const String &rhs) {
 }
 
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-String & String::operator =(String &&rval) {
+String & String::operator =(String &&rval) noexcept {
     if (this != &rval)
         move(rval);
     return *this;
 }
 
-String & String::operator =(StringSumHelper &&rval) {
+String & String::operator =(StringSumHelper &&rval) noexcept {
     if (this != &rval)
         move(rval);
     return *this;

cores/esp8266/WString.h

@@ -57,8 +57,8 @@ class String {
         String(const String &str);
         String(const __FlashStringHelper *str);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-        String(String &&rval);
-        String(StringSumHelper &&rval);
+        String(String &&rval) noexcept;
+        String(StringSumHelper &&rval) noexcept;
 #endif
         explicit String(char c);
         explicit String(unsigned char, unsigned char base = 10);
@@ -96,8 +96,8 @@ class String {
         String & operator =(const char *cstr);
         String & operator = (const __FlashStringHelper *str);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-        String & operator =(String &&rval);
-        String & operator =(StringSumHelper &&rval);
+        String & operator =(String &&rval) noexcept;
+        String & operator =(StringSumHelper &&rval) noexcept;
 #endif
 
         // concatenate (works w/ built-in types)
@@ -317,7 +317,7 @@ class String {
         String & copy(const char *cstr, unsigned int length);
         String & copy(const __FlashStringHelper *pstr, unsigned int length);
 #ifdef __GXX_EXPERIMENTAL_CXX0X__
-        void move(String &rhs);
+        void move(String &rhs) noexcept;
 #endif
 };